Larvae of Musca domestica were exposed to spores of Nosema algerae on the surface of their diet. Infective concentrations (IC50 and IC90) for the larvae were 3.6 x 10(4) and 1.6 x 10(6) spores/cm2, respectively. The disease appeared to cause no larval mortality, but the longevity of adult females was reduced. At 30 days post-infection, there were at least 1 x 10(7) spores per fly in all dosage groups. At lower dosages, the development of spores was delayed and fewer spores were produced.