Epizootics of the microsporidium Thelohania solenopsae were monitored in Louisiana populations of red imported fire ants, Solenopsis invicta. This included a natural epizootic in a monogyne ant population (single-queen colonies), the first report of infections found in this social form in the field. Colonies in a 0.2-ha plot near Baton Rouge, Louisiana, were 1.0 : 0 monogyne: polygyne in June, 1999, 0.78 : 0.22 in February, 2000, and 1.0 : 0 in May, 2001. The epizootic began with three colonies infected in October, 1999, and peaked in February, 2000, when 17% of monogyne and 80% of polygyne colonies were infected. Prevalence decreased until May, 2001, when the last infected colony was detected. The microsporidium failed to infect colonies when it was artificially released in a monogyne population. Three releases of T. solenopsae in polygyne populations (multiple-queen colonies) have had variable success. The microsporidium spread rapidly within six months at one release site and required almost two years to begin spreading at a second; it has become well established at these two sites. At a third polygyne site, T. solenopsae became established but then slowly died out.