The mating-type locus regulates choice of mating partner and the formation of mixed-mating-type heterokaryons inNeurospora crassa. These biological activities reflect the effects of the mating-type genes on several distinct biochemical pathways. Some elements of such pathways are probably common to many other fungi. An example of such a pathway would be the mating-type-specific pheromone production and response known inN. crassa whose components inSaccharomyces cerevisiae are well characterized. The mating-type locus has two alternative forms or idiomorphs. Themta idiomorph apparently encodes a single product, the MT a-1 polypeptide. The structure and DNA-binding activity of MT a-1 suggest that it is a transcriptional activator of specific mating-related genes. Vegetative incompatibility depends on a different mechanism of action because requirements for this activity can be separated from those for mating. Themt A idiomorph encodes three products. MT A-1 is likely to be a transcriptional regulator similar toS. cerevisiae α 1. MT A-2 and MT A-3 are likely to be transcriptional regulatory proteins active after fertilization. The mating-type genes show surprising cross-species activity, and are not likely to be primary species barriers within the genusNeurospora. The role of other components of the mating pathways in speciation and in life history evolution remain to be tested.