- Noah Beery Jr.
- Jan Trevor
- Dorothy Short
- Mona Andrews
- Harry Woods
- Borno
- Bryant Washburn
- Dr. Harry Trevor
- Walter Miller
- Dr. Frank Bracken
- Fred MacKaye
- Dr. Charles Phillips
- John Davidson
- Prince Samu [Chs. 11-12]
- J. Frank Glendon
- Speaker at Conference [Ch. 1]
- Viva Tattersall
- Georgia Trevor [Ch. 1]
- Gwendolyn Logan
- Mrs. Henry Andrews
- William Desmond
- Allen
- Grace Cunard
- Mrs. Camerford Amster [Ch. 1]
- Russ Powell
- Henry Andrews
- Don Brodie
- Dr. Carl Neff [Ch. 1]
- Harry Worth
- Dr. Carl Neff
- Dickie Jones
- Jan Trevor as a Boy [Ch. 1]
- Eddie Kane
- Gen. Tichak [Ch. 12]
- Stanley Andrews
- Emperor Mena [Ch. 12]
- Chicma the Chimpanzee
- Jan's Chimpanzee Playmate
- Richard Alexander
- Navigator
- King Baggot
- Dr. Pierce [Ch. 1]
- H. Burroughs
- Kutu
- Al Ferguson
- Guard of Mu [Chs. 10-12]
- Jack Leonard
- Gorilla [Chs. 7-8]
- Eric Mayne
- Doctor at Conference [Ch. 1]
- J.P. McGowan
- Freighter Captain
- Charles Murphy
- Grubb, Freighter's Mate [Ch. 1]
- Eddie Parker
- Emperor's Guard [Ch. 11] / Mu Sentry [Ch. 12] (uncredited)
- Buddy Roosevelt
- Batu [Ch. 12]
- Hal Taliaferro
- Lancer Officer-in-Charge [Chs. 10-12]
