The bioautographic technique for detecting fungitoxic compounds on thin-layer chromatograms was modified for use with species of Phycomycetes by using Phytophthora capsici, P. megasperma var. sojae and Pythium aphanidermatum as examples. The modification makes possible the detection and assay of materials specifically fungitoxic to Phycomycetes and may be of value in the demonstration of naturally occurring antifungal compounds in diseases caused by Phycomycetes. Methods are described for preparing suspensions of encysted zoospores and spraying these onto chromatograms in dilute V-8 juice agar. Fungitoxic spots were visualized by immersing the plates in a suspension of charcoal in water. The charcoal adhered to the mycelium, but not to the smooth agar surface of the inhibition zones. The method was illustrated by bioassaying the fungicides metalaxyl, furalaxyl and propamocarb, which are specific for Phycomycetes, and the phytoalexins, glyceollin and capsidiol. With P. megasperma var. sojae, 25 ng of metalaxyl could be detected. P. aphanidermatum, but not the Phytophthora spp., was sensitive to propamocarb. Thus, even within the Phycomycetes, selection of appropriate species for bioassay may be important.