- Billy Halop
- Billy Adams
- Huntz Hall
- Toby Nelson
- Gabriel Dell
- Bilge
- Bernard Punsly
- Butch
- Hal E. Chester
- Swab
- Joe Recht
- Lug
- William Hall
- Brack Warren
- John McGuire
- Tom Adams
- Mary Field
- Aggie Nelson
- Edward Keane
- Elliott Carlton [Chs. 1-7]
- Marcia Ralston
- Leah Carlton
- Reed Hadley
- Carl Tonjes
- Stanley Blystone
- Capt. Olaf Nelson
- Richard Alexander
- Jenkins
- Ernie Adams
- Zeke - Skipper of the 'Mary Lou'
- Jack Rube Clifford
- Pug Anderson [Chs. 1-6]
- Richard Bond
- Krans, Agent X-39 [Chs. 4, 6]
- Morgan Wallace
- Capt. Lester [Chs. 1-3, 6-7]
- Eddie Dunn
- Whaler Captain [Chs. 8-9]
- James Blaine
- Desk Sergeant [Ch. 7]
- Ed Cassidy
- Guard at Hospital [Ch. 6]
- Jack Cheatham
- Duke [Ch. 1]
- Dick Curtis
- Mate on the 'Astoria' [Chs. 4-5]
- Larry Grenier
- Island Guard
- Al Hill
- Speeding Motorist [Ch. 2]
- Jack Holmes
- (uncredited)
- Bill Hunter
- Policeman [Chs. 3-4]
- Jerry Jerome
- 'Astoria' Officer [Ch. 4]
- Guy Kingsford
- Sven - Whaler Crewman [Ch. 9]
- Jerry Mandy
- Tony - Grocer [Chs. 1-2]
- John Merton
- First Island Guard [Chs. 10-11]
- William Moss
- (uncredited)
- Jack Mulhall
- 'Dolphin' Radioman [Chs. 1-2]
- Paul Newlan
- Chris - Second Island Guard [Chs. 11-12]
- Eddie Parker
- Coral Strand Crewman [Chs. 6-9]
- Jack Perrin
- Island Guard-Launch Pilot [Ch. 12]
- House Peters Jr.
- Agent Burke [Ch. 4]
- Joe Pope
- (uncredited)
- Anthony Rogers
- Jim - Police Boat Pilot [Chs. 2, 6]
- Hans Schumm
- Sub Captain [Chs. 9, 11-12]
- George Sherwood
- Island Guard
- Mickey Simpson
- Angry Motorist [Ch. 2]
- Tom Steele
- Coral Strand Crewman [Ch. 6] / Whaler Crewman [Chs. 8-9] (uncredited)
- Robert R. Stephenson
- (uncredited)
- Forrest Taylor
- Fenwick the Inventor [Chs. 1, 5, 9, 12]
- Frank Wayne
- Fenwick's Guard [Ch. 12]
- Duke York
- Pete - Whaler Crewman [Chs. 8-9]
