Various mutants have been obtained of B. brongniartii, a hyphomycete pathogenic for insects. Both one pigment less and two blastospore negative mutants retained their initial virulence. Thus neither the characteristic parasitic blastospore morphology nor the synthesis of the pigment appeared to be required for pathogenesis. Analysis of avirulent mutants indicated that chitinolytic activity may be essential for fungal penetration of the cuticle.