A mating type analysis was performed on 231 isolates of the cellular slime mold, Polysphondylium pallidum found in 61 samples collected in eastern North America between northern Florida and sourthern Canada. Seventy-eight percent of the isolates belonged to one of 2 mating types; 18% were incapable of mating with any partner; 3% were homothallic; and 1%, consisting of 2 isolates from a Florida sample, belonged to a separate breeding group. It is suggested that the majority of isolates represent a species capable of local genetic adaptation to a niche, the parameters of which undergo considerable variation over space and time.