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)
