Tim Holt as Lt. Cmdr. John 'Twill' Twillinger

Audrey Dalton as Gail MacKenzie

Hans Conried as Dr. Jess Rogers

Barbara Darrow as Jody Simms

Max Showalter as Dr. Tad Johns

Harlan Warde as Lt. Robert 'Clem' Clemens

Gordon Jones as Sheriff Josh Peters

Mimi Gibson as Sandy MacKenzie

Marjorie Stapp as Connie Blake

Jody McCrea as Seaman Fred Johnson

Wallace Earl as Sally

Dennis McCarthy as George Blake (lab assistant)

Sarah Selby as Mrs. Simms

Milton Parsons as Lewis Clark Dobbs

Charles Tannen as Seaman Wyatt (Radioman)

Robert Benevides as Seaman Morty Beatty

John Carlyle as Monster Victim

John Close as Deputy Larry

William Forrest as Admiral Greenhouse

Gil Frye as Deputy Scotty

Joseph Hamilton as Other Lock Watchman

Charles Herbert as Boy with Morty's Cap

Byron Kane as Coroner Nate Brown

Ralph Littlefield as Gatekeeper

David McMahon as Patterson

Ralph Moody as Watchman at Lock 57

William Swan as Seaman Howard Sanders

Hal Taggart as Davis - Resort Owner

Mack Williams as Captain Masters

Dan Gachman as Deputy Brewer

Michael Dugan as Clarke