A STRIPED LIONFISH, ITS TENTAcles grazing a branch of live coral, swims lazily with dozens of exotic companions in a 500-gallon tank behind the bar of Jimmy Johnson's waterfront Florida Keys home. It's the perfect pet for a man who has never had time for animals that need to be walked, brushed or scratched behind the ears. But these days even an aquarium can be too much trouble for the veteran coach. "I put myself totally into what I'm doing," says Johnson. And right now what he's doing isn't fish; it's football.

While the tropical specimens are cared for by a fish-sitter, Johnson, 53, is plotting his assault on the NFL as the new head coach of the Miami Dolphins. After Super Bowl wins with the Dallas Cowboys in 1993 and '94, followed by two years as a TV commentator, Johnson now Wants to make winners of the underachieving Dolphins, and his quest could provide one of the season's most compelling sideshows.

Johnson's return to pro football brings him back to the scene of his first coaching triumph. In the 1987 season, he won a national championship as head coach of the University of Miami Hurricanes. Two years later he moved to Dallas, where his bulldog, in-your-face style transformed the 1-15 Cowboys into two-time world champions. Then came the famous clash of egos as Cowboys owner Jerry Jones, who'd been a teammate of Johnson's in college, claimed that any coach could have won with the team Jones had assembled. Johnson subsequently quit.

Last January, he got a call from Dolphins owner (and Blockbuster Video founder) H. Wayne Huizenga, who was seeking a successor to retiring Don Shula. First, Huizenga wanted Johnson to convince him that he hadn't softened during his two years off. According to SPORTS ILLUSTRATED, Johnson told him, "I need to win." That was enough for Huizenga, who is reportedly paying Johnson $8 million for four years.

It came as no surprise to Johnson-watchers that he refused to deliver the ceremonial pat on the back to his predecessor during the Dolphins' annual awards dinner last April. "I'm supposed to say congratulations to all the people in the past, to all the great tradition," Johnson said, all but ignoring Shula, 66, who had guided the team for 26 years, winning Super Bowls in 1973 and '74. "But I only care about one thing: the present—the people who are here now to win now."

"I don't know whether he loves winning or hates losing more," says his friend and attorney Nick Christin. After his UM team lost to Penn State in the 1986 Fiesta Bowl, Johnson collapsed in tears. And on a charter flight following a Dallas loss to the Redskins in 1992, Johnson got so angry at a player who laughed that he told everyone, including the flight attendants, to "sit their asses down."

"What drives me," says the coach, "is going through the absolute misery of losing and, on the other side, experiencing the ecstasy of being on the players' shoulders after winning. I don't want my players to take losing lightly. I want them to be sick to their stomachs, because the more miserable they are, the more it will motivate them." Says his older son, Brent, 32, a Dallas attorney: "He absolutely does not care what other people think, but he takes it personally when things he felt he had control over go wrong."

Johnson has been known to lighten up)—especially during his two-year break from football when he fished off the Florida Keys and tossed down Heinekens with his live-in girlfriend, Rhonda Rookmaaker, 42. In 1994 he bought a lavish home in the Keys' Tavernier, along with Wave Runners and scuba gear. He also bought a 48-foot yacht, Three Rings, referring to his three championship rings, where Johnson has occasionally let down his well-lacquered hair to demonstrate his moonwalk or to chow down a batch of nachos. When Johnson informed Rookmaaker that he was taking the Dolphins job, she burst into tears. "I'd had him all to myself for two years," she says. Now, he's "all focus, all football. He loves it as much as fishing. I told him I understood he had to do it one more time."

For Johnson, of course, any gridiron is home turf. The second of three children of a dairy superintendent and his wife, James William Johnson was born in blue-collar Port Arthur, Texas. A decent student, he was a defensive lineman for the Arkansas Razorbacks, and some teammates dubbed him Jimmy Jumpup for the way he'd bounce back after being knocked flat.

Intending to pursue a career as an industrial psychologist, Johnson took a temporary job after graduation as defensive coordinator at Louisiana Tech University. "I was married and had a son, and they were offering $1,000 a month, a car and an apartment—a lot of money in 1965," he explains.

Johnson had discovered his calling, and it soon became his obsession—with his family playing second-string. Son Chad, 29, a Dallas financier who played high school football, says the first time his father saw him in action was during his senior year when he had an away game in Michigan at the same time as Johnson's Miami Hurricanes. "He caught the last quarter," says Chad.

That dedication had its price. In 1989 Johnson had been married to his college sweetheart, the former Linda Kay Cooper, for 26 years when, according to his autobiography, Turning the Thing Around, he came home after jogging one day and said, "I want a divorce." (Johnson says he has seen his ex-wife once since then, in 1992, at Brent's wedding. She now teaches grade school in Helsinki, Finland.) Observers speculated that the Cowboys job had come between Johnson and his wife. But he says, "We were growing apart, and our interests were in different areas."

Once he was single again, Johnson insisted he wanted to live the rest of his life as a bachelor, but Rookmaaker changed all that. A former Miami hairstylist who met Johnson 10 years ago when he came in for a cut and a manicure, Rookmaaker continues to shape pro football's most recognizable hairdo. ("He kind of enjoys how people make fun of his hair," she says.) She moved in with him in 1994 just before he left Dallas, and though the pair have no plans to marry, Rookmaaker wears a seven-carat diamond cocktail ring, a gift from Johnson, who is known for occasional extravagances. (He once plopped down a $5,000 gambling chip in front of an assistant during a postseason team trip to the Bahamas.) "She is my best friend," he says. But Rookmaaker has yet to hear the words "I love you" from the usually talkative coach. "We don't do that," she explains.

One reason the relationship works, Rookmaaker says, is that "I know his moods." When she asks Johnson about the guest list for his coach's suite at Miami's Pro Player Park and he doesn't answer because he's thinking about tomorrow's practice, she says, "I know it's time to go into another room."

Since the spring, she and Johnson have spent most weeks in a two-bed-room condominium on exclusive Williams Island, which is much closer to the Dolphins' facilities than their Keys home. Three Rings is docked in the adjacent marina but isn't taken out much. "What's important," says Johnson, "is being happy. And I'm happy when I'm accomplishing something."

With the NFL season under way (the Dolphins won their opener), the old intensity is back. On the sidelines, Johnson weaves in and out of clusters of Dolphin players, shouting and patting. But behind the exhortations is an iron will. "I'm not only hard on those who don't work hard and do the things I ask," says Johnson, "I eliminate them."

Fear may be a motivator, but Johnson's enthusiasm comes through too. Says Randal Hill, a Dolphins wide receiver who also played for him at the University of Miami: "I wouldn't be surprised to see coach Johnson turn out for practice in full dress—shoulder pads, everything!"

Almost everything, anyway. A helmet would just ruin that famous coif.