 | John Agar als Prof. Clete Ferguson |
 | Lori Nelson als Helen Dobson |
 | John Bromfield als Joe Hayes |
 | Nestor Paiva als Captain Lucas |
 | Grandon Rhodes als Jackson Foster |
 | Dave Willock als Lou Gibson |
 | Robert B. Williams als George Johnson |
 | Charles Cane als Police Captain |
 | Loretta Agar als Woman on Boat (uncredited) |
 | Bill Baldwin als Patrol Boat Dispatcher (voice / uncredited) |
| Jere Beery als Photographer (uncredited) |
| Ricou Browning als The Gill Man (in water) / Lab Technician (uncredited) |
 | Diane DeLaire als Miss Abbott (uncredited) |
| Mike Doyle als Cop (uncredited) |
 | Clint Eastwood als Jennings (uncredited) |
| Jack Gargan als Skipper (uncredited) |
| Charles Gibb als Cop (uncredited) |
 | Brett Halsey als Pete (uncredited) |
 | Don C. Harvey als Mac - Spotlight Party Leader (uncredited) |
| Tom Hennesy als The Gill Man (on land) / Marineland Diver (uncredited) |
| Don House als Diver (uncredited) |
 | Robert F. Hoy als Charlie (uncredited) |
| Ned Le Fevre als Newscaster (uncredited) |
| Sydney Mason als Police Announcer (uncredited) |
| Robert Nelson als Dr. McCuller (uncredited) |
| Patricia Powers als Girl in Convertible |
 | Bert Stevens als Search Party Member (uncredited) |
| Charles Victor als Policeman (uncredited) |
| Robert Wehling als Joe (uncredited) |
| James R. Wolfe als Man at Tank (uncredited) |
| Jack Arnold | Director |
| Jack Kevan | Creature Design |
| Scotty Welbourne | Director of Photography |
| William Alland | Story |
| Martin Berkeley | Screenplay |
| Paul Weatherwax | Editor |
| William Alland | Producer |
| Alexander Golitzen | Art Direction |
| Russell A. Gausman | Set Decoration |
| William Lava | Original Music Composer |
| Herman Stein | Original Music Composer |
| Alfred Sweeney | Art Direction |
| Julia Heron | Set Decoration |
| Bud Westmore | Makeup Artist |
| Jack A. Bolger, Jr. | Sound |
| Leslie I. Carey | Sound |
| Ben Hendricks | Sound Editor |
| Albert E. Kennedy | Sound Editor |
| Jay A. Morley Jr. | Costume Designer |
| Joan St. Oegger | Hairstylist |
| Edward Dodds | Unit Production Manager |
| Fred Frank | Assistant Director |
| Tom Hennesy | Stunts |
| Don House | Stunts |
| Bob Hoy | Stunts |
| Ginger Stanley | Stunts |
| Joseph Gershenson | Music Supervisor |
| Henry Mancini | Music |
| Milton Rosen | Music |
| Ethmer Roten | Musician |
| Hans J. Salter | Music |
| Frank Skinner | Music |
