The a mating type locus of Ustilago maydis contains the structural genes for a pheromone-based cell recognition system that governs fusion of haploid cells. Binding of pheromone to its cognate receptor includes mating competence in haploid cells and stimulates filamentous growth of the dikaryon. We have analyzed transcription of genes located in the a locus and demonstrate that all genes are induced by pheromone. Transcriptional stimulation is mediated by a 9 bp DNA element (ACAAAGGGA) that occurs in multiple copies in both alleles of the a locus. By fusing multimers containing this 9 bp sequence to the pheromone gene promoter and to a heterologous promoter we demonstrate that this sequence acts as a pheromone response element. In addition, we show that expression of the b genes, which regulate pathogenic development of the dikaryon, is also stimulated by pheromone. Pheromone-inducible genes can be divided into three classes depending on whether their expression is reduced, maintained, or increased after cell fusion. These differences may suggest some regulatory cross-talk between the a and b loci.