Craig Foster
Sheila Layton
The Purple Monster
Dr. Cyrus Layton
Hodge Garrett
Marcia [Chs. 11-13]
Emperor of Mars [Chs. 2, 10-11]
Carl Stewart
Saunders [Chs. 3-4]
Dr. Paul Meredith [Ch. 9] (as George Carleton)
Charles Mitchell [Chs. 1-2, 10]
Helen / Marcia posing as Helen [Chs. 11-12]
Dr. Harvey [Chs. 13-14]
Prof. Benjamin [Ch. 11]
Prof. Crandall (as Frederick Howard)
Tony [Ch. 7]
Andy Martin [Ch. 8]
Police Inspector Evans (uncredited)
Radio Room Technician (uncredited)
Shaw (uncredited)
Barnes (uncredited)
Baker / Curry / Ed Fletcher / Logan / Lab Thug (uncredited)
Mack (uncredited)
Clay (uncredited)
Narrator / Radio Broadcaster (voice) (uncredited)
Lab Thug / Fritz Benham / Meers (uncredited)
Cop / Ambulance Driver (uncredited)
Dr. Harvey's Assistant (uncredited)
Osborne / Mason (uncredited)
