After 15 weeks as the No. 1 film in the nation, ''Titanic'' was finally toppled at the box office over the weekend by ''Lost in Space,'' a science-fiction movie based on the 1960's television series.

The new film grossed an estimated $20 million, far ahead of ''Titanic,'' the James Cameron blockbuster, which took in about $11.6 million over the weekend. In third place was ''Mercury Rising,'' a Bruce Willis action film, which grossed an estimated $10.9 million, a disappointment to its studio, Universal.

'' 'Titanic' is finally dethroned -- bound to happen one day,'' John Krier, president of the Exhibitor Relations Company, which monitors the box office, said this morning. He said, '' 'Lost in Space,' had a tremendous amount of publicity, a lot of hoopla.''

The big-screen adaptation of ''Lost in Space,'' which cost about $75 million and is the most expensive film ever made by New Line, a division of Time Warner, is based on the 1965-68 CBS science-fiction series about a 21st-century astrophysicist who takes his family on a trip through the cosmos. The film version received unenthusiastic reviews from critics.