SEVERAL workers1–5 have postulated the production of some non-enzymatic phytotoxins by Rhizoctonia solani Kuhn, a pathogen having a wide range of plants as hosts; but the chemical constitution of the toxins is yet unknown. Recently, Sherwood and Lindberg6 reported that this toxin was a phenolic and glycosidic substance from the results of paper chromatogram and biological tests; but no substance was identified on the paper. We have already reported that the culture filtrate of R. solani gave phenylacetic acid, a well-known plant growth regulator, and the acid could inhibit the growth of roots of seedlings at concentrations above 0.05 per cent for sugar beet, and 0.005 per cent for rape and rice plant respectively7. Phenylacetic acid, however, did not cause necrosis, which was a characteristic phenomenon of the root rot, therefore it seemed likely that the acid was not the principal phytotoxin produced by this fungus. In this communication, a further investigation on isolating phytotoxins is shown.