The toxic effect of fernasan (containing 25% thiram) was tested on Rhizoctonia solani and Fusarium solani in liquid and agar media. The fungicide was more effective in liquid (at 100 p.p.m.) than in solid media, where 400 p.p.m. checked the growth of R. solani, but was ineffective against F. solani. Both fungi exhibited a zone of inversion growth in liquid culture whereby certain intermediate concentrations were less inhibitory than lower or subsequently higher doses. Similar reaction was recorded for F. solani alone in fungicide-containing agar media. Sclerotium formation by R. solani was highly significant, in relation to controls, at 40 p.p.m. The abundance of chlamydospores of F. solani was coupled with cessation of conidium formation with increasing fernasan doses.