 | Ralph Byrd | ... | Lt. Terry Kent |

| Bela Lugosi | ... | Boroff, alias M.A. Anderson |
 | Maxine Doyle | ... | Jean Norman |
 | Richard Alexander | ... | Thorg |
 | Lee Ford | ... | Snapper McGee |
 | Herbert Rawlinson | ... | Cmdr. Boyle |
 | John Picorri | ... | G.A. Rackerby |
 | Lawrence Grant | ... | Rabinisi (Boroff's spokesman at screening) |
 | Thomas Carr | ... | Ens. Jim Kent |
 | Carleton Young | ... | Henchman Dodds [Ch. 4] |
 | Allen Connor | ... | Dick Norman |
 | George Chesebro | ... | L.H. DeGado - Chief Thug |
 | Ranny Weeks | ... | Wies - Ambulance Orderly thug |
| rest of cast listed alphabetically: |
 | Roy Barcroft | ... | Goebel, Henchman [Chs. 2, 10] (uncredited) |
 | Richard Beach | ... | Seaman (uncredited) |
 | Earle D. Bunn | ... | Kelp Plant Heavy 1 [Chs. 8-9] (uncredited) |
 | Yakima Canutt | ... | Seaman (uncredited) |
 | Ed Cassidy | ... | Patrol Boat Capt. Johnson [Chs. 1-3, 5, 12] (uncredited) |
 | Jack Clifford | ... | Carver (uncredited) |
 | Baldwin Cooke | ... | Seaman (uncredited) |
 | Floyd Criswell | ... | Motorcycle Heavy [Ch. 8] (uncredited) |
 | Jack Daley | ... | Captain of the S.S. 'Carfax' [Ch. 1] (uncredited) |
 | Leon Davidson | ... | Seaman (uncredited) |
 | Forrest Dillon | ... | Sea Wolf Thug [Ch. 7] (uncredited) |
 | Lester Dorr | ... | Hospital Radium-Tech [Ch. 11] (uncredited) |
 | Curley Dresden | ... | Dock Heavy 1 [Chs. 6-7, 10] (uncredited) |
 | Robert Dudley | ... | Pierport Station Agent [Ch. 6] (uncredited) |
 | Norwood Edwards | ... | Seaman (uncredited) |
 | Frank Ellis | ... | Kelp Plant Worker [Ch. 8] (uncredited) |
 | Frank Fanning | ... | Roadside Police Sergeant [Ch. 3] (uncredited) |
 | Jerry Frank | ... | Sea Wolf Thug [Chs. 3, 5, 7] (uncredited) |
 | Lee Frederick | ... | Citizen [Ch. 3] (uncredited) |
 | Audrey Gaye | ... | Telephone Switchboard Supervisor [Ch. 2] (uncredited) |
 | Joseph W. Girard | ... | Cmdr. Green [Ch. 1] (uncredited) |
 | Kit Guard | ... | Kelp Plant Heavy 2 [Chs. 8-9] (uncredited) |
 | Henry Hale | ... | Krohn - Morovanian Scientist (uncredited) |
 | Jack Ingram | ... | Seaman [Chs. 2-3, 6-7] (uncredited) |
 | Warren Jackson | ... | Boat Heavy #4 [Ch. 5] (uncredited) |
 | Payne B. Johnson | ... | Boy in lifeboat (uncredited) |
 | Jerry Larkin | ... | Henchman (uncredited) |
 | Rex Lease | ... | Coast Guard Orderly [Chs. 1, 3] (uncredited) |
 | Alexander Leftwich | ... | Froman, Morovanian emissary [Ch. 1] (uncredited) |
 | Jack Long | ... | Seaman (uncredited) |
 | Joseph P. Mack | ... | Captain of the S.S. 'Adamic' [Chs. 5-6] (uncredited) |
 | Teddy Mangean | ... | Seaman (uncredited) |
 | Gene Marvey | ... | Blake (uncredited) |
 | Frankie Marvin | ... | Sniper [Ch. 7] (uncredited) |
 | Frank Meredith | ... | Policeman [Ch. 6] (uncredited) |
 | James Millican | ... | Dock Heavy 4 [Chs. 6-7, 10] (uncredited) |
 | Pat Mitchell | ... | Radioman (uncredited) |
 | King Mojave | ... | Ambulance Driver-Thug [Chs. 3-4] (uncredited) |
 | Edwin Mordant | ... | Dr. Meade [Chs. 6, 11] (uncredited) |
 | Michael Morgan | ... | Coast Guardsman Moore [Chs. 3, 11-12] (uncredited) |
 | Henry Morris | ... | Dock Heavy 2 [Chs. 6-7, 10] (uncredited) |
 | Charles Murphy | ... | S. S. 'Adamic' Mate [Chs. 5-6] (uncredited) |
 | John Mustin | ... | Policeman (uncredited) |
 | Henry Otho | ... | Mate on the S.S. 'Carfax' [Ch. 1] (uncredited) |
 | Eddie Phillips | ... | Boat Heavy 1 [Ch. 5] (uncredited) |
 | Loren Riebe | ... | Pete, Thug [Chs. 5, 8] (uncredited) |
 | Vinegar Roan | ... | Dock Heavy 3 [Ch. 6-7, 10] (uncredited) |
 | Jack Roberts | ... | Jones - Racherby's Lab Assistant [Ch. 2] (uncredited) |
 | Buddy Roosevelt | ... | Patrol Boat Lt. Scott (uncredited) |
 | Dick Scott | ... | Gang Launch Pilot [Ch. 1] (uncredited) |
 | Reed Sheffield | ... | Thug Loading Launch [Ch. 11] (uncredited) |
 | Dick Sheldon | ... | Ambulance Attendant-Thug [Chs. 3-4] (uncredited) |
 | Harry Strang | ... | Phone Company Code Expert [Ch. 2] (uncredited) |
 | Duke Taylor | ... | Loader [Ch. 6] (uncredited) |
 | Tom Ung | ... | Charlie, 'Sea Wolf' Cook [Ch. 7] (uncredited) |
 | Billie Van Every | ... | Telephone Operator [Ch. 2] (uncredited) |
 | Robert Walker | ... | Henchman Black [Chs. 6-7, 10] (uncredited) |
 | Frank Wayne | ... | Henchman Payne [Chs. 3, 5, 7-8, 10-11] (uncredited) |
 | Herbert Weber | ... | Belden, Cargo Clerk-Thug [Ch. 5] (uncredited) |

| Robert J. Wilke | ... | Seaman (uncredited) |
 | Robert Williams | ... | Sloan, Henchman [Ch. 8] (uncredited) |
 | Roger Williams | ... | Black, a Thug [Chs. 6-7, 10] (uncredited) |
 | Dan Wolheim | ... | Sea Wolf Captain Slauson [Ch. 7] (uncredited) |
 | Duke York | ... | Sea Heavy #1 [Ch. 5] (uncredited) |