The concentrations of thiram preventing germination of spores ofBotrytis cinereain drops of a 1% solution of sucrose, and on the surface of a sucrose‐nitrate agar have been determined. Thiram had much less effect on germination in the agar medium, even when a purified agar was used. There was no growth on sucrose‐nitrate agar if the concentration of thiram exceeded 31 p.p.m. Attempts to obtain strains able to grow at higher concentrations were unsuccessful.Similar results were obtained with ziram, nabam and zineb.Ferbam also was more effective in preventing spore germination in spore drops than on agar media; this effect was obtained with ordinary and with purified agar.On a sucrose‐nitrate agar generally there was no growth if the concentration of ferbam exceeded 125 p.p.m., but in one of forty‐eight plates containing 250 p.p.m. ferbam, five slowly growing colonies were produced, and from these colonies arose mycelium which grew and sporulated rapidly on 500 p.p.m. ferbam agar. Agar disk inocula were transferred from these cultures to agar containing higher concentrations of ferbam and in this way, and by repeating the process, a strain was obtained which grew slowly but continuously, and sporulated on agar containing 5000 p.p.m. ferbam. However, the poor solubility of this fungicide made it difficult to assess quantitatively the degree of adaptation.A proportion of the spores from this strain germinated in drops containing about twice the concentration of ferbam which prevented germination of parent spores.The resistance of the mycelium of the resistant strain was not lost after repeated subculture on fungicide‐free agar. The resistant strain was as susceptible as the parent strain to thiram, ziram, nabam and zineb.Attempts to obtain strains ofVenturia inaequalisresistant to thiram, ferbam, ziram and zineb were uns