- Mala
- Agent Ray Mala
- Rex
- Rex - the Wonder Horse
- Buck the Dog
- Buck
- Mamo Clark
- Princess Melani
- Herbert Rawlinson
- Grant Jackson
- William Newell
- Hank McGlaurie
- John Ward
- Anthony Tupper
- John Dilson
- E.G. Ellsworth
- Selmer Jackson
- Mr. Canfield
- John Picorri
- Porotu
- George Chesebro
- Draker - Island Operations Chief
- Bob Kortman
- Wilson - Gang Yacht Captain
- George Cleveland
- Goebel - Gang Scientist
- Lloyd Whitlock
- Lamar - Headquarters Gang Member [Chs. 1-6]
- Tiny Roebuck
- Eppa - the Wrestler
- Tracy Layne
- Larkin - Gang Chalkboard Monitor
- Herbert Weber
- Stevens
- Anthony Pawley
- Clark - Dying Man on Clipper Island
- Allen Connor
- Jim Taylor - Infiltrator [Chs. 10-11]
- Frazer Acosta
- Teekar - Porotu's Chief Henchman
- Don Brodie
- Dirigible 'San Francisco' Radioman [Chs. 1, 11]
- Ed Cassidy
- Joe Davis - Intelligence Chief [Ch. 10]
- Allan Cavan
- Captain of Dirigible 'California' [Chs. 13-14]
- Edmund Cobb
- Crosby / Harris (uncredited)
- Lester Dorr
- Dirigible 'California' Officer [Ch. 14]
- Val Duran
- Half-Caste Drummer
- Frank Ellis
- Rontree - Sailor-Thug in Stocking Cap
- Roscoe Gerald
- Price - Cave Thug in Visor
- Henry Hale
- Mercer - Cave Thug at Desk
- Oscar 'Dutch' Hendrian
- Grover - Gang Headquarters Majordomo [Ch. 12]
- F. Herrick Herrick
- Tollar - Sub Captain (Ch. 2,3,4,10-13 - 14)
- John David Horsley
- Grover's Co-pilot / Agent Ed Varne (uncredited)
- Jerry Jerome
- Wallace - Dirigble Henchman [Ch. 13]
- Cactus Mack
- Fosdick / Manning (uncredited)
- Charles Marsh
- Dirigible 'San Francisco' Asst. Radioman [Chs. 1, 11]
- Allen Mathews
- Agent M-2 [Ch. 11]
- Ralph McCullough
- Goebel's Lab Assistant
- Charles McMurphy
- Inspector O'Hara [Chs. 10-11]
- Bud Osborne
- Ellis - Mate
- Eddie Phillips
- Ellsworth's Male Secretary [Chs. 1, 9-10]
- Loren Riebe
- Johnson - Seaman [Ch. 7]
- Buddy Roosevelt
- Wireless Operator
- Jack Stewart
- Police Sergeant [Ch.10]
- Harry Strang
- Grover - Gang Pilot [Chs. 4-6]
- Henry Sylvester
- Fairchild - Captain of Dirigible 'San Francisco' [Chs. 1, 11]
- Al Taylor
- Marco - Sailor-Thug in Officer's Hat
- Evan Thomas
- Agent 1
- Larry Thompson
- Agent 2
- Francis Walker
- Spy
