Liam Redmond
as Professor Mark O'Brien
Peggy Cummins
as Joanna Harrington
Dana Andrews
as Dr. John Holden
Niall MacGinnis
as Dr. Julian Karswell
Percy Herbert
as Farmer (deleted from US print)
Charles Lloyd Pack
as Chemist
Reginald Beckwith
as Mr. Meek
Peter Hobbs
as Superintendent
Maurice Denham
as Professor Henry Harrington
Athene Seyler
as Mrs. Karswell
Ewan Roberts
as Lloyd Williamson
Peter Elliott
as Professor K.T. Kumar
Rosamund Greenwood
as Mrs. Meek
Richard Leech
as Inspector Mottrarn
Lloyd Lamble
as Detective Simmons
John Salew
as Librarian