Margaret O'Brien
— Alpha
James Craig
— Mike Regan
Marsha Hunt
— Katie Mallory
Philip Merivale
— Professor Peter Vincent
Henry O'Neill
— Professor Josh Pringle
Donald Meek
— Professor Catty
Keenan Wynn
— Packy Roost
Alan Napier
— Dr. Woodring
Sara Haden
— Rhoda Kitterick
Kathleen Lockhart
— Mrs. Catty
Howard Freeman
— Professor Richards
Walter Fenner
— Professor Endicott
Elisabeth Risdon
— Mrs. Pringle
Robert Blake
— Jerry
Barbara Bedford
— Telephone Operator (uncredited)
Ava Gardner
— Hat Check Girl (uncredited)
Mike Mazurki
— Fighter (uncredited)
Pauline Starke
— Bobby's Mother (uncredited)
Eddie Acuff
— Policeman (uncredited)
Hooper Atchley
— Pepi (uncredited)
William Bishop
— Reporter (uncredited)
Naomi Childers
— Matron (uncredited)
Victor Sen Yung
— Chinese Man (uncredited)