Two strains of the insect pathogenic fungus Enfomophfhora virulenta have been examined for their pathogenicity to adult blowflies, Calliphora erythrocephala, for their ability to grow on defined media, and for the production of biologically active secondary metabolites. The fungus proved highly pathogenic to blowflies and hyphal bodies isolated from diseased insects were successfully cultivated on egg yolk medium and subsequently grown in shake culture on a glucoseasparagine medium. Both strains were found to produce a mixture of 4,4'-azoxybenzene dicarboxylic acid and 4,4'-hydroxymethyl azoxybenzene carboxylic acid. The hydroxy acid proved toxic by intrahemocoelic injection into adult blowflies and accounted for all the extractable insecticidal activity.