SummarySeveralAscosphaeraspecies were tested for their pathogenicity for larvae of the alfalfa leafcutting bee,Megachile rotundata.Larvae were reared aseptically on a sterile pollen-based diet and inoculated at 6 days of age with spores of the fungi.A. aggregatacaused the distinctive syndrome called‘chalk brood’. BothA. apisandA.proliperdawere pathogenic for larvae and produced syndromes similar to chalk brood inApis mellifera.Chalk brood inM. rotundatawas characterized by colour changes and sporulation ofA. aggregalabeneath the host cuticle.A. apisandA. proliperdacaused no colour changes in infected larvae; white mycelia erupted through the cuticle before sporulation occurred.A. atraandA. majorgrew saprophytically on provisions of leafcutting-bee cells and did not cause larval disease. Spores taken from larvae killed byA. apisor A.proliperdagrew less readily on the pollen diet, and appeared to be more virulent, than spores taken from subcultures.