ABSTRACTThe family Radiomycetaceae, which includes two genera,HesseltinellaandRadiomyces, is based, in part, on the production of a unique sporangiolum-bearing structure consisting of a once-septate stalk terminated by a vesicle bearing one or many uni- or multispored sporangiola. Zygospores, where known, have in common a smooth zygosporangial wall and opposed, appendage-bearing suspensors.Hesseltinello vesiculosa, the only known species of the genus, produces its stalked sporangiolum-bearing structures in lateral, subterminal, or terminal clusters. Only one sporangiolum is formed per fertile vesicle; sporangiolar appendages are acicular with flattened, hexagonal bases, and zygospores are unknown. The fungus is presumably heterothallic. InRadiomycesthe sporangiolum-bearing branches always arise terminally on the enlarged apex of the sporangiophore and many sporangiola are formed per fertile vesicle; sporangiolar appendages are capitate or spathulate; zygospores are as described above, and the taxa are homothallic. A new species,Radiomyces mexicana, is described. This species resemblesR. embreeibut differs in having: 1) somewhat smaller spore-bearing structures, 2) globose to obpyriform (not sphaeroidal) fertile vesicles, 3) relatively thick-walled sporophore vesicles and fertile vesicles, 4) a slower rate of sporangiospore germination, and 5) a lower upper temperature range for good growth. On certain media a variable percentage of zygosporangia are ovoid to ellipsoid and the sporophore often has a pronounced subsporangial constriction. A cladistic analysis revealed that the genera of Radiomycetaceae and the family are each monophyletic.Radiomyces embreeiandR. mexicanaare more closely related to one another than toR. spectabilisbecause they are unispored (a synapomorphy).Radiomyces embreeiandR. mexicanaare characterized by autapomorphies and are considered monophyletic. Keys to the genera and species are provided.