Fusarium moniliforme and Cunninghamella elegans were grown in fluid Sabouraud medium at 28 C. They were dosed with 6.2 mM phthalazine and then incubated for 3 d with shaking at 125 rpm. After extraction with ethyl acetate, the metabolites were purified by HPLC and identified by the UV/visible, mass, and NMR spectra. The results show that F moniliforme oxidized about 42% of the phthalazine to 1(2H)-phthalazinone (= 1-hydroxyphthalazine) and that C. elegans oxidized about 6% of the phthalazine to a novel metabolite, phthalazine N-oxide (= phthalazine 2-oxide).