In this paper we describe the cloning of the DNA region containing the A1 mating type genes of the secondarily homothallic mushroom Coprinus bilanatus and compare its organization to that of heterothallic homobasidiomycetes. As in other species, the C. bilanatus A factor contains several different genes that encode two different types of homeodomain transcription factor (HD1 and HD2); and some of these genes are active in the heterologous host C. cinereus. The HD1 and HD2 genes are distributed over two closely linked subloci, Aalpha and Abeta. A gene coding for a mitochondrial intermediate peptidase (mip) directly flanks the Aalpha sublocus. The pab-1 gene, required for para-aminobenzoic acid synthesis, is found 39 kb upstream of mip. The structural arrangement of this chromosomal region closely resembles the heterothallic C. cinereus. In contrast, the Aalpha and Abeta subloci of Schizophyllum commune are further separated, with pab-1 located between the two subloci, suggesting that a translocation event may have occurred during evolution.