This project is about improving understnding of mechanisms for sexual reproduction in the ascomycete class Laboulbeniomycetes. With few exceptions, members of this class are obligate parasies on arthropods. Nominal species are often specific to a narrow range of host species, and sometimes even to specific positions on the body of the parasite. In the extreme case, several species of the same genus inhabit different parts of the body of the same host. We are examining one such case in the genus Coremyces, the members of which appear on the wings and belly of coroxid beetles. We have already confirmed that tere are five different species with different positional preferences on the host, in some case also with a preference for one of the host sexes. These five species appear to be closely related, which gives rise to a series of question s how species boundaries arise and are maintained. One essential piece of information missing is about the processes of sexual reproduction: Can the fungus mate with itself, or is outcrossing with another individual necessary? To that end, we have produced five annotated genmes from members of Coremyces, the primary use of which is to locate the mating-type (MAT) loci regulating sexual compatibility among individuals of the same species. We intend to locate these loci in order to be able to understand the reproductive mechanisms and thereby, by extension, how different closely related species may have evolved on the same host.