 Hugh MarloweDr. Russell A. Marvin | Dr. Russell A. Marvin |
 Joan TaylorCarol Marvin | Carol Marvin |
 Donald CurtisMaj. Huglin | Maj. Huglin |
 Morris AnkrumBrig. Gen. John Hanley | Brig. Gen. John Hanley |
 Thomas Browne HenryVice Adm. Enright | Vice Adm. Enright |
| GGrandon RhodesGen. Edmunds | Gen. Edmunds |
 John ZarembaProf. Kanter | Prof. Kanter |
 Paul FreesAlien (voice) | Alien (voice) |
 Frank WilcoxAlfred Cassidy | Alfred Cassidy |
| SSydney MasonFrank the garage man | Frank the garage man |
 Larry J. BlakeMotorcycle cop | Motorcycle cop |
| BBert StevensAir Intelligence Command Officer | Air Intelligence Command Officer |
 Forbes MurrayMilitary Officer at Experiment | Military Officer at Experiment |
 Arthur ToveyOfficer/Civilian at Military Conference | Officer/Civilian at Military Conference |
| AAlan WellsMajor Kimberly | Major Kimberly |
| NNicky BlairMilitary Officer at Experiment (uncredited) | Military Officer at Experiment (uncredited) |
| JJimmy CrossMilitary Messenger (uncredited) | Military Messenger (uncredited) |
| JJack DeeryMilitary Officer (uncredited) | Military Officer (uncredited) |
 Charles EvansDr. Alberts (uncredited) | Dr. Alberts (uncredited) |
| RRaoul FreemanMilitary Official (uncredited) | Military Official (uncredited) |
| Paul FreesAlien (voice) (uncredited) | Alien (voice) (uncredited) |
| JJames GonzalezMilitary Official (uncredited) | Military Official (uncredited) |
| DDuke GreenMinor Role (uncredited) | Minor Role (uncredited) |
| EEd HaskettMilitary Official (uncredited) | Military Official (uncredited) |
 Clark HowatSgt. Nash (uncredited) | Sgt. Nash (uncredited) |
 Harry LauterCutting - Generator Technician (uncredited) | Cutting - Generator Technician (uncredited) |
| DDon MarloweMinor Role (uncredited) | Minor Role (uncredited) |
| WWilliam MeaderWorker in Control Tower (uncredited) | Worker in Control Tower (uncredited) |
 Sol MurgiPlane Passenger (uncredited) | Plane Passenger (uncredited) |
| Forbes MurrayMilitary Officer at Experiment (uncredited) | Military Officer at Experiment (uncredited) |
