The microsporidium, Thelohania solenopsae, (Microsporida: Thelohaniidae) is a pathogen that causes the slow demise of a fire ant colony. It is one of the most common pathogens of fire ants in Brazil and Argentina. In 1996, during a survey for pathogens in imported fire ant colonies in the U.S., we discovered this microsporidium in worker ants of the red imported fire ant in Florida, Mississippi and Texas. Fire ant queens in infected colonies weighed 50% less than queens in noninfected colonies. Laboratory studies indicate that infected colonies had significant reductions in worker ants, immatures stages, queen weights, and egg laying rates. In the first field study initiated in Florida in 1997, T. solenopsae has spread extensively and resulted in 62% maximum reduction in fire ant populations. Field inoculations of T. solenopsae were initiated in 1998 and 1999 in small plots in 10 southern states and infections have been detected in 7 of the 10 states. In addition, we have seen limited spread after 1 to 1 ½ years. Also, T. solenopsae infections were found in 80% of the winged reproductives collected from mating flights. In 1998, the first monogyne colonies were inoculated in a field site in Florida and infections were detected and the disease had spread to other colonies at the site in 1999. This was the first evidence of spread of T. solenopsae in a monogyne population of fire ants. The impact of this pathogen on imported fire ant populations in the U.S. is being extensively studied. In addition, both laboratory and field studies are underway with another microsporidian, Vairimorpha invictae. When this pathogen is added to a colony infected with T. solenopsae, the result is a quicker demise of fire ant colonies. Research with both pathogens is continuing in the U.S. and in Argentina.