Helminth parasites of rats have been investigated in many parts of the world; however, no previous investigations of the parasites of this host have been made on the Aleutian Islands. Because rats are implicated in the transmission of various diseases to man, a knowledge of rat parasites is fundamental. The purpose of the present study was to obtain information concerning the helminth parasites of Adak rats and to determine, insofar as possible, the occurrence of related health problems. This study was conducted on Adak during May 1950 in conjunction with animal-borne disease investigations by the Arctic Health Research Center, U.S. Public Health Service, Anchorage, Alaska.