 John AgarProf. Clete Ferguson | Prof. Clete Ferguson |
 Lori NelsonHelen Dobson | Helen Dobson |
 John BromfieldJoe Hayes | Joe Hayes |
 Nestor PaivaCaptain Lucas | Captain Lucas |
 Grandon RhodesJackson Foster | Jackson Foster |
 Dave WillockLou Gibson | Lou Gibson |
 Robert B. WilliamsGeorge Johnson | George Johnson |
 Charles CanePolice Captain | Police Captain |
 Loretta AgarWoman on Boat (uncredited) | Woman on Boat (uncredited) |
 Bill BaldwinPatrol Boat Dispatcher (voice / uncredited) | Patrol Boat Dispatcher (voice / uncredited) |
| JJere BeeryPhotographer (uncredited) | Photographer (uncredited) |
 Ricou BrowningThe Gill Man (in water) / Lab Technician (uncredited) | The Gill Man (in water) / Lab Technician (uncredited) |
 Diane DeLaireMiss Abbott (uncredited) | Miss Abbott (uncredited) |
| MMike DoyleCop (uncredited) | Cop (uncredited) |
 Clint EastwoodJennings (uncredited) | Jennings (uncredited) |
 Jack GarganSkipper (uncredited) | Skipper (uncredited) |
| CCharles GibbCop (uncredited) | Cop (uncredited) |
 Brett HalseyPete (uncredited) | Pete (uncredited) |
 Don C. HarveyMac - Spotlight Party Leader (uncredited) | Mac - Spotlight Party Leader (uncredited) |
| TTom HennesyThe Gill Man (on land) / Marineland Diver (uncredited) | The Gill Man (on land) / Marineland Diver (uncredited) |
| DDon HouseDiver (uncredited) | Diver (uncredited) |
 Robert F. HoyCharlie (uncredited) | Charlie (uncredited) |
| NNed Le FevreNewscaster (uncredited) | Newscaster (uncredited) |
| SSydney MasonPolice Announcer (uncredited) | Police Announcer (uncredited) |
| RRobert NelsonDr. McCuller (uncredited) | Dr. McCuller (uncredited) |
| PPatricia PowersGirl in Convertible | Girl in Convertible |
| BBert StevensSearch Party Member (uncredited) | Search Party Member (uncredited) |
| CCharles VictorPoliceman (uncredited) | Policeman (uncredited) |
| RRobert WehlingJoe (uncredited) | Joe (uncredited) |
| JJames R. WolfeMan at Tank (uncredited) | Man at Tank (uncredited) |
