The soil fungi Mortierella pusilla, M. isabellina, and Trichoderma viride, and the aquatic fungus Saprolegnia parasitica were chosen as test organisms to study the fungitoxicity of the pesticides methoxychlor and fenitrothion and some of their known metabolites and derivatives. The inhibition of mycelial growth on agar plates was used as a criterion for toxicity and it was established that the inhibiting effect was fungistatic rather than fungicidal. The results, which indicate that some metabolites are more toxic than the parent pesticides, are reported in the form of dosage response relationships and a novel method of reporting such relationships is proposed.