Determination of the polarity of Psilocybe coprophila Fr. was made, using two strains, K and T, collected in Japan. This fungus has been suggested to include both the bi- and tetrapolar strains.Judging only from the results of test for clamp connections, but ignoring the difference in the number of them, in all possible pairings of monosporous mycelia isolated from a f ruitbody, this fungus may be said to be bipolar. The clamp-bearing mycelia can, however, be subdivided into two groups. The one is characterized by many clamp connections, producing normal fruitbodies, and the other by only a few clamp connections, producing haploid fruitbodies bearing spores of the same mating type. The latter group can hardly be regarded as real diploid mycelia, because of the appearance of haploid fruitbodies which often develop on the monosporous mycelia of this fungus. From this point of view, the polarity of this fungus is thought to be tetrapolar.In the pairings of monosporous mycelia, when both A- and B-factors are different, the number of clamp connections is many, but when A-factors, but not B- factors, are different, it is few, and when B-factors, but not A-factors, are different, or when both A- and B-factors are identical, no clamp connection is formed.It was found that one common factor, B1, was involved in the two strains which were collected from .two places about 500km apart from each other.The regular formation of clamp connections, though it is few in number, in the illegitimate combinations in which B-factors, but not A-factors, are identical, and also the existence of one common factor, B1, in the remote places seem to suggest the evolutional course from bipolarity to tetrapolarity in this fungus.