Adiaspores of the fungus Emmonsia crescens were detected microscopically in the lung tissue of 13% of 10,081 small mammals belonging to 24 species examined in 14 areas of the Czech Republic between 1986 and1997; 441/1,934 (23%) Rattus norvegicus). The overall prevalence of emmonsiosis was significantly higher in adult (19%) than in juvenile (7%) mammals, and in rodents (13%, and 20% in adults) than in insectivores (2%, and 4% in adults). The frequency of infected mammals also varied according to geographic area, altitude, habitat, and season.