| Robin Williams (Professor Philip Brainard) | Marcia Gay Harden (Dr. Sara Jean Reynolds) | Christopher McDonald (Wilson Croft) |
| Raymond J. Barry (Chester Hoenicker (as Raymond Barry)) | Clancy Brown (Smith) | Ted Levine (Wesson) |
| Wil Wheaton (Bennett Hoenicker) | Edie McClurg (Martha George) | Jodi Benson (Weebo) |
| Leslie Stefanson (Sylvia) | Malcolm Brownson (Father) | Benjamin Brock (Window Boy) |
| Dakin Matthews (Minister) | Zack Zeigler (Teenage Boy) | Samuel Lloyd (Coach Willy Barker) |
| Scott Michael Campbell (Dale Jepner) | Bob Sarlatte (Rutland Coach) | Bob Greene (Referee) |
| Tom Barlow (Medfield Basketball Player) | Scott Martin Gershin (Flubber (voix)) | Julie Morrison (Weebette (voix)) |
| Adryenn Ashley (Basketball Fan (scenes deleted)) | Ulf Kallvik (Wesson (Swedish version) (voix)) | Sam Lloyd (Coach Willy Barker) |
| Scott Bowe (Garbage Man (non-crédité)) | Chris Chapo (Basketball Fan (non-crédité)) | Teresa Couch (Basketball Fan (non-crédité)) |
| Ray Doherty (Basketball Fan (non-crédité)) | Erica Engelhardt (Basketball Fan (non-crédité)) | Moya Huffman (Student (non-crédité)) |
