Trichomycetes (Zygomycota) are arthro- pod-associated fungi consisting of three orders, only one of which, the Harpellales, contains cultured spe? cies. Isozyme patterns were studied using starch gel electrophoresis of 108 isolates representing 18 de? scribed species from six genera of Harpellales (Cap- niomyces, Furculomyces, Genistelloides, Simuliomyces, Smittium, and Trichozygospora), and some unde? scribed species of Smittium, as well as three isolates of Amoebidium parasiticum (Amoebidiales) and three species of nontrichomycetous fungi. A total of 176 different band positions for 13 loci in 11 enzyme sys? tems were produced. The banding patterns were con? sistent with haploidy. Numerical taxonomic and prin- cipal coordinate analyses of Trichomycetes compared isozyme variation within and among described and undescribed species, including three named isolates of uncertain identity. There was no discernible pat? tern among the cultures attributable to geographic origin or type of insect host family. Isolates of de? scribed species produced similarity clusters that gen? erally correlated well with morphologically defined genera and species. Based on isozyme patterns, one isolate tentatively identified as G. hibernus was re-ex- amined and reclassified as C. stellatus. Thirty-six un? described isolates of Smittium produced seven clus? ters that probably represent new species. Other un? described Smittium isolates grouped with described species and may be conspecific. Cophenetic correla? tion coefficients for 13 clusters of Harpellales species were >0.90, but for A. parasiticum it was only 0.87; the latter may represent a species complex. Thirty- two isolates of Sm. culisetae originating from France, Australia, Japan, and six USA states (including Ha? waii) were isozymically very similar or identical to each other, despite differences in their geographic origins and dipteran host families. A phenogram based on isozyme patterns generally corresponded with one from a previous immunological study.