microsporidian infection in a natural population of mosquitofish, Gambusia affinis (Baird and Girard)

. A microsporidian parasite was found in a natural population of mosquitofish, Gambusia affinis, from southern California. The parasite, a Glugea sp, induced the formation of xenoma complexes in the mesenteries of infected fish. In cases of severe infection, the xenomas occupied the majority of the abdominal cavity and appeared to prevent efficient swimming. This report is an initial documentation of this infection and a description of some of the clinical signs observed.