WHEN INVESTOR JEFF MILLER moved to Marina del Rey eight years ago, he introduced himself to Tommy Rettig, who lived next door in a one-bedroom waterfront condo. Hearing the name, Rettig seemed surprised. "What?" asked the new neighbor. "You know another Jeff Miller?"

"Yeah," Rettig answered. "I played him on Lassie."

With time, there were fewer of those reminders for Rettig, who was 54 when he died of heart failure Feb. 15. Since 1985, the former actor had run a successful, home-based business, designing computer software for universities and government agencies. But he received several letters a week from fans who still thought of him as the smiling boy who, from 1954 to 1957, called, "Here, girl!" to America's quadruped sweetheart. Less happily, other people remembered Rettig as one of those child stars who'd drifted into trouble. In a string of jobs he had little luck, and he was busted in the '70s for marijuana possession and cocaine smuggling. (The latter charge was overturned.)

Although he was for years an outspoken advocate of recreational drug use, the substance he may have abused the most was fat. Death, ruled a coroner, was brought on by arteriosclerosis. Ex-wife Darlene, 51, a friend since their 1976 divorce, says, "Anything that would clog the arteries, he'd eat it." He used to love steak and junk food.

Rettig, the only child of Elias Rettig, a Lockheed aircraft-parts inspector, and his wife, Rosemary, began his career at age 5, after he was spotted by an acting coach who lived in the family's apartment building in Queens, N.Y. After touring with Mary Martin in Annie Get Your Gun, he landed roles in movies, among them 1954's River of No Return, starring Marilyn Monroe. Then, at 12, he was cast as Jeff Miller. He bonded strongly with his canine costar—who, like the whole line of Lassies, was male—and even took him home on weekends (the family had moved West in 1949). That stopped when Lassie became confused about whether to obey his trainer or Rettig.

After four seasons, Rettig was deemed too old, and Lassie was given a new, younger master, 7-year-old Jon Provost. Rettig later told friends his final episode was the happiest day of his life. "I wanted to be a real kid," he said. Recognized everywhere, though, he abandoned that idea. After graduating from L.A.'s University High in 1958 and marrying 15-year-old Darlene Portwood, he tried to get back into acting, without much success. While he took pride in being the father of two sons, Tom and Duane, now 35 and 34, Rettig later said that, in his 20s, "I considered suicide every day."

In the early '80s, having tried selling tools and managing a health club, he founded an est-like motivation program. But it wasn't until he sat down at a computer to compile a mailing list that he finally found himself. "He was fascinated but not mystified" by computers, says software designer Tom Rombouts, who worked for him.

Rettig remained friends with Provost, who would always end their visits together with a line he had spoken in Rettig's last episode: "Thanks for the dog, Jeff!" In the end, though, Lassie may have had the final word. The collie now filling the role (there's always some Lassie project) was expected to attend Rettig's memorial in Marina del Rey.

TOM CUNNEFF in Los Angeles