Variations of enzymatic activity, pH and mycelium growth during life cycle of representatives of Zoophthora and Paecilomyces species and correlation between these properties, were described. Apart from monoamino oxalate and low molecular carboxylic acids, any other compounds which could be considered as mycotoxines were not found. The enzymatic activity seems to be the main factor in the mechanism of insect infection and intoxication by both analyzed fungus species.