Margaret O'Brien as Alpha

James Craig as Mike Regan

Marsha Hunt as Katie Mallory

Philip Merivale as Professor Peter Vincent

Henry O'Neill as Professor Josh Pringle

Donald Meek as Professor Catty

Keenan Wynn as Packy Roost

Alan Napier as Dr. Woodring

Sara Haden as Rhoda Kitterick

Kathleen Lockhart as Mrs. Catty

Howard Freeman as Professor Richards

Walter Fenner as Professor Endicott

Elisabeth Risdon as Mrs. Pringle

Robert Blake as Jerry

Barbara Bedford as Telephone Operator (uncredited)

Ava Gardner as Hat Check Girl (uncredited)

Mike Mazurki as Fighter (uncredited)

Pauline Starke as Bobby's Mother (uncredited)

Eddie Acuff as Policeman (uncredited)

Hooper Atchley as Pepi (uncredited)

William Bishop as Reporter (uncredited)

Naomi Childers as Matron (uncredited)

Victor Sen Yung as Chinese Man (uncredited)