"In a tributary of the Amazon, a monster – half-man, half-fish – is captured and placed in a reservoir in a Florida national park to be observed by scientists."
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
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)
Jack Arnold
Director
Martin Berkeley
Screenplay
William Alland
Story
Scotty Welbourne
Director of Photography
Jack Kevan
Creature Design
Producer
William Lava
Original Music Composer
Herman Stein
Paul Weatherwax
Editor
Alexander Golitzen
Art Direction
Alfred Sweeney
Russell A. Gausman
Set Decoration
Julia Heron
Bud Westmore
Makeup Artist
Jack A. Bolger, Jr.
Sound
Leslie I. Carey
Ben Hendricks
Sound Editor
Albert E. Kennedy
Jay A. Morley Jr.
Costume Designer
Joan St. Oegger
Hairstylist
Edward Dodds
Unit Production Manager
Fred Frank
Assistant Director
Stunts
Ginger Stanley
Joseph Gershenson
Music Supervisor
Henry Mancini
Music
Milton Rosen
Ethmer Roten
Musician
Hans J. Salter
Frank Skinner