The pathogenic dimorphic fungal organism Blastomyces dermatitidis exists as a budding yeast at 37 degrees C and as a mycelium at 25 degrees C. While the conversion of one morphological phase of B. dermatitidis to another has long been known to be a thermally dependent process, little of the accompanying biochemical or genetic events controlling the phase transition has been elucidated. Using differential cDNA library screening, we have identified one transcript, bys1, in B. dermatitidis that is expressed at very high levels in the yeast phase but whose levels diminish rapidly when yeast cells are transferred to 25 degrees C to promote conversion to the mycelial phase. Although the 0.95-kb bys1 transcript is absent in B. dermatitidis mycelia maintained at 25 degrees C, transfer of mycelial cultures to 37 degrees C results in the reappearance of bys1 within 12 h. bys1 codes for a protein of 18.6 kDa that contains multiple putative phosphorylation sites, a hydrophobic N terminus, and two 34-amino-acid domains with similarly spaced nine-amino-acid degenerative repeating motifs. Although the nature of the thermal dependency of bys1 expression and the function of the bys1 protein are unknown, the strong expression of this transcript specifically in the yeast phase of B. dermatitidis may prove to be very useful in the development of more specific and sensitive diagnostic methods for blastomycosis.