The ability of 18 compounds to inhibit morphogenesis and/or growth of the diploid stage of Allomyces macrogymus was assayed. Of the three stages of development -- germination, vegetative growth, and sporogenesis -- germination appeared to be the most sensitive to inhibition and vegetative growth the least sensitive. Sporogenesis was sensitive to most of the inhibitors tested, but high concentrations were required for effective activity. Inhibitors could be grouped according to the stage of germination (cyst or rhizoid) at which they blocked development; those effective at the rhizoid stage, could be divided further on the basis of the resultant morphology of the germling.