Of the 5 isolates of P. polycephalum studied, each displayed a different pair of mating factors, and 4 of the pairs were reported for the 1st time. The 10 mating types are assumed to constitute a single multiple allelic series. However, because 1 of the 5 isolates (N.C.) consistently yielded only nonsporulating hybrid plasmodia when crossed with the other 4, genetic analyses of these F1 generations could not be made. On the other hand, interisolate hybrid plasmodia from the remaining combinations sporulated, and F1 results were in conformity with a 1-locus, multiple allelic model.