John Agar as Prof. Clete Ferguson

Lori Nelson as Helen Dobson

John Bromfield as Joe Hayes

Nestor Paiva as Captain Lucas

Grandon Rhodes as Jackson Foster

Dave Willock as Lou Gibson

Robert B. Williams as George Johnson

Charles Cane as Police Captain

Loretta Agar as Woman on Boat (uncredited)

Bill Baldwin as Patrol Boat Dispatcher (voice / uncredited)

Jere Beery as Photographer (uncredited)

Ricou Browning as The Gill Man (in water) / Lab Technician (uncredited)

Diane DeLaire as Miss Abbott (uncredited)

Mike Doyle as Cop (uncredited)

Clint Eastwood as Jennings (uncredited)

Jack Gargan as Skipper (uncredited)

Charles Gibb as Cop (uncredited)

Brett Halsey as Pete (uncredited)

Don C. Harvey as Mac - Spotlight Party Leader (uncredited)

Tom Hennesy as The Gill Man (on land) / Marineland Diver (uncredited)

Don House as Diver (uncredited)

Robert F. Hoy as Charlie (uncredited)

Ned Le Fevre as Newscaster (uncredited)

Sydney Mason as Police Announcer (uncredited)

Robert Nelson as Dr. McCuller (uncredited)

Patricia Powers as Girl in Convertible

Bert Stevens as Search Party Member (uncredited)

Charles Victor as Policeman (uncredited)

Robert Wehling as Joe (uncredited)

James R. Wolfe as Man at Tank (uncredited)