A new species ofPseudoarachniotus, P. hyalinosporus, is described and illustrated. A total of 54 isolates of this species was studied and their growth was compared on several media. Although certain differences appeared in colonial coloration and growth characteristics, all strains were similar in 2 fundamental characteristics, viz., type of ascospore and type of gametangial initials and their subsequent development. The initials in early stages are often so closely appressed one to another that they give the appearance of a single structure. Further development takes place by the elongation of the ascogonium which forms a loop around the male initial at right angles to the linear plane of that initial. The ascospores ofP. hyalinosporus are small, about 2.2 × 3.3µ, thin-walled, oval, hyaline to very pale yellow, and smooth, thereby differing fromPseudoarachniotus reticulatus which has globose, echinulate-reticulate ascospores, andP. roseus andP. citrinus which have large, thick-walled, oval to oblate ascospores. Furthermore, the colonies ofP. roseus are red, those ofP. citrinus are yellow, and inP. hyalinosporus colonies are white with pale yellow asciferous areas and usually with a great deal of green pigment both above and reverse. Of the 54 strains of this new species, 25 were isolated from rodent lungs, 9 from dung, 19 from soil and one from a tinea pedis condition.Pseudoarachniotus hyalinosporus was found in two collections from India and it appears to be widely distributed in California.