Amblyospora bracteata, Polydispyrenia simulii, and 3 other undescribed microsporidia are reported from 11 species of neotropical blackfly larvae collected from streams in Buenos Aires (eastern) and Neuquen (southwestern) provinces of Argentina. Preliminary light and electron microscopy studies indicate that 2 of the undescribed species belong in the family Thelohaniidae and the third species is placed in the family Caudosporidae. Ten species of Argentine blackflies are recorded as new host records for A. bracteata and P. simulii.