The distribution and relatedness of long, surface, proteinaceous fibrils (fimbriae) in a number of species in the Ustilaginales and related heterobasidiomycetous yeast were studied using EM and an antifimbrial antibody prepared against a pure fimbrial preparation from a1 cells of Ustilago violacea. Only one examined species (Tilletia caries) from the family Tilletiaceae lacked fimbriae. All species in the Ustilaginaceae and the heterobasidiomycetous yeasts produced long fimbriae similar in size and shape to those of U. violacea. The cells from all of the tested species in the Ustilaginaceae were agglutinated by this antibody and appear to have a similar or identical protein subunit in their fimbriae. The temperature range in which fimbriae were produced varied among species and among physiologic races of U. violacea. None of the tested heterobasidiomycetous yeasts was agglutinated by the U. violacea antibody, and the protein subunit in their fimbriae is a distinct type. Cells of either mating type a1 or a2 and diploid a1a2 cells of U. violacea were agglutinated by the antibody prepared against fimbriae from a1 cells. Although fimbriae appear to have a role in cell communication during conjugation, the evidence suggests that the fimbriae of each mating type are identical rather than sex specific and either perform a purely mechanical function or are modified in a sex-specific manner.