which function as sporidia. Before the parasitic dikaryophase can be established, fusions between haplonts of opposite compatibility (plasmogamy) must occur. In the process of teliospore maturation, the nuclei of those dikaryotic cells destined to become teliospores fuse (karyogamy). At maturity each teliospore thus contains a single diploid nucleus. Although there are exceptions, promycelia of bipolar species typically produce both A and a haplonts, since factors which govern compatibility segregate during reduction division. Smuts in which compatibility is governed by 2 allelic pairs (tetrapolar species) produce four kinds of sporidia, AB, Ab, aB, ab. In either case, as a normal consequence of plasmogamy, karyogamy, and reduction division, haplonts of two or four compatibility groups result. A review of sexuality and attendant pathogenicity in the smut fungi is not intended here (for a critical review of sexuality in the smut fungi see Whitehouse, 1949, 1951, 1954; Fischer and Holton, 1957) ; nevertheless, a brief summarization of salient points dealing with sexuality in the smut fungi seems justified because sexual phenomena in the onion smut fungs, Urocystis colchici (Schlecht.), Rabenh. (U. cepulae Frost) are apparently at variance with those in other smuts. Our data support the thesis that, contrary to other smuts, the teliospores of U. colchici function as unisexual entities. This phenomenon, reported by Tachibana and Duran (1961), is not fully understood at present, nor are other anomalies, e.g., the disproportionate numbers of teliospores of one mating type from single telia. This paper presents