Bradford Dillman
as Paul Grogan

Heather Menzies
as Maggie McKeown

Kevin McCarthy
as Dr. Robert Hoak

Keenan Wynn
as Jack

Dick Miller
as Buck Gardner

Barbara Steele
as Dr. Mengers

Belinda Balaski
as Betsy

Melody Thomas Scott
as Laura Dickinson

Bruce Gordon
as Colonel Waxman

Barry Brown
as Trooper

Paul Bartel
as Mr. Dumont

Shannon Collins
as Suzie Grogan

Shawn Nelson
as Whitney

Richard Deacon
as Earl Lyon

Janie Squire
as Barbara Randolph

Roger Richman
as David

Bill Smillie
as Jailer

Guich Koock
as TV Pitchman

Jack Pauleson
as Boy in Canoe

Eric Henshaw
as Father in Canoe

Robert Vinson
as Soldier

Virginia Dunnam
as Girl

Diamond Farnsworth
as Water-skier

Bruce Paul Barbour
as Man in Boat

Robyn Ray
as Screaming Woman

Mike Sullivan
as Dam Guard

Jack Cardwell
as Brandy

Sally Bondi
as Swimming Deb (uncredited)

Eric Braeden
as Dr. Robert Hoak (swimming double) (uncredited)

Joe Dante
as Scuba Diver #2 (uncredited)

Amy Holden Jones
as Hertz Clerk (uncredited)

Andrew G. La Marca
as Scuba Diver #1 (uncredited)

John Sayles
as Army Sentry (uncredited)

Phil Tippett
as Scuba Diver (uncredited)