Published in Astounding Science Fiction, July 1949. Collected in
An alien species of 'noncorporeal parasites' called the Ceetal are engaged in a systematic conquering of the galaxy. These creatures can take over a human body and rule it from the inside and they have already inhabited the leaders of nearly a thousand star systems. Zone Agent Iliff is sent to the planet Gull where Zone Agent Trainee Pagadan, a non-human telepath, has uncovered the plot and together they must find a way to stop it.