John Agar

Prof. Clete Ferguson

Lori Nelson

Helen Dobson

John Bromfield

Joe Hayes

Nestor Paiva

Captain Lucas

Grandon Rhodes

Jackson Foster

Dave Willock

Lou Gibson

Robert B. Williams

George Johnson

Charles Cane

Police Captain

Loretta Agar

Woman on Boat (uncredited)

Bill Baldwin

Patrol Boat Dispatcher (voice / uncredited)

Ricou Browning

The Gill Man (in water) / Lab Technician (uncredited)

Diane DeLaire

Miss Abbott (uncredited)

Clint Eastwood

Jennings (uncredited)

Brett Halsey

Pete (uncredited)

Don C. Harvey

Mac - Spotlight Party Leader (uncredited)

Robert F. Hoy

Charlie (uncredited)

Bert Stevens

Search Party Member (uncredited)