AbstractIn the filamentous oomycete fungusAchlya, the differentiation of gamete bearing structures on vegetative hyphae of the male mating type, is induced by theAchlyasteroid hormone, antheridiol. Among the several metabolically labeled intracellular proteins whose synthesis or accumulation is altered by hormone treatment are steroid‐induced 85‐kDa and 68‐ to 78‐kDa proteins. The 85‐kDa protein was previously shown to be theAchlyaheat shock protein hsp85 [Bruntet al., 1990; Brunt and Silver, 1991], a component of the putativeAchlyasteroid hormone receptor. It was of interest to determine if the antheridiol‐induced “70‐kDa” proteins were hsp70‐family heat shock proteins and if hormone treatment‐induced changes in the level of hsp70 transcripts. Two differentAchlyahsp70 genomic sequences were cloned and used to investigate these questions. The two hsp70 sequences recognized two different mycelial transcript populations, one of which was regulated also by decreased glucose. Of note, both of the two hsp70 transcript populations were found to be regulated by antheridiol. The hormone‐induced chcnges in hsp70 transcript levels were temporally correlated with the onset of massive lateral hyphal branching and alterations in the pattern of secreted N‐linked glycoproteins which occur in hormone‐treated mycelia. To our kncwledge, this represents one of the first reports on changes in hsp70 proteins and transcripts during fungal differentiation. Our results may have implications for the role of heat shock proteins in hyphal branching and secretion in filamentous fungi and perhaps other cell types.