ABSTRACTTwo triazoles, cyproconazole and propiconazole, and the imidazole, prochloraz, all commercially available fungicides that inhibit 14α-demethylation during sterol synthesis, were applied to a cholesterol-producing fungus,Allomyces macrogynus, during growth phase. Concentrations of the above inhibitors 1/100-1/500 of the EC50concentrations needed for inhibition of vegetative growth, caused failure of sporangia to cleave normally and produce uniformly sized, uniflagellate zoospores. Instead, dependent upon the concentration of fungicide used, motile cells were uniflagellate, biflagellate, triflagellate or multiflagellate with the number of nuclei corresponding to the number of flagella. The fungicides were only effective if they were present during the growth phase and impairment of cleavage could not be overcome by adding cholesterol to the discharge medium. Observations were based on light and electron microscope observations. Multinucleate cells with more than one flagellum germinated to produce normal thalli when plated on agar lacking the fungicides. The basis of the interference with cleavage is discussed.