The structure, homologies, polymorphism and expression profiles of a new gene, aap1, have been studied for precisely characterizing it and defining its putative involvement in thermo-tolerance of both vegetative mycelium growth and sporophore differentiation. Sequence polymorphism was analyzed in 3 homokaryons of A. bisporus and 24 strains having different abilities for mycelial growth at temperatures above 30°C and for producing mature fruiting bodies at 25°C. The level of gene expression was measured by real-time PCR both in vegetative mycelium after transfer from 25 to 32°C and in primordia and fruiting bodies produced during cultures at 17 or 25°C. The results indicated that aap1 gene belong to a new subfamily of the yeast YAP1 homologs. It is not a dominant contributor to the thermo-tolerance of A. bisporus, but the protein it encodes may be involved as an overall stress resistance transcription factor. The way Aap1 senses redox level differs from that of AP-1-like transcription factor Yap1.