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)

Jere Beery

Photographer (uncredited)

Ricou Browning

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

Diane DeLaire

Miss Abbott (uncredited)

Mike Doyle

Cop (uncredited)

Clint Eastwood

Jennings (uncredited)

Jack Gargan

Skipper (uncredited)

Charles Gibb

Cop (uncredited)

Brett Halsey

Pete (uncredited)

Don C. Harvey

Mac - Spotlight Party Leader (uncredited)

Tom Hennesy

The Gill Man (on land) / Marineland Diver (uncredited)

Don House

Diver (uncredited)

Robert F. Hoy

Charlie (uncredited)

Ned Le Fevre

Newscaster (uncredited)

Sydney Mason

Police Announcer (uncredited)

Robert Nelson

Dr. McCuller (uncredited)

Patricia Powers

Girl in Convertible

Bert Stevens

Search Party Member (uncredited)

Charles Victor

Policeman (uncredited)

Robert Wehling

Joe (uncredited)

James R. Wolfe

Man at Tank (uncredited)