The a mating-type region of Neurospora crassa controls several major events in both the sexual and asexual phases of the fungal life cycle. This 3235-base-pair DNA segment is not homologous to the comparable genetic region of the A mating type. The unique a and A regions are bordered by nearly identical DNA sequences. The a genetic region contains at least two functional segments. One segment encodes a perithecium maturation function that is dependent on the second segment for phenotypic expression. This second a segment encodes a spliced mRNA that specifies the mt a-1 polypeptide. This polypeptide appears to be responsible for vegetative incompatibility, mating identity, and perithecium induction. The a-1 transcript is produced vegetatively and under conditions that induce sexual differentiation. The amino-terminal half of the mt a-1 polypeptide is homologous to the shorter Schizosaccharomyces pombe mat-Mc polypeptide. This homology and the properties of mt a-1 mutants suggest that the a-1 polypeptide segment that is homologous to the mat-Mc polypeptide may be primarily responsible for mating functions, while the distal segment is required for vegetative incompatibility.