On water agarose conidial germination of Ampelomyces quisqualis was greatly enhanced in the presence of powdery mildew conidia. All seven species of powdery mildews tested, representing five genera, were effective in increasing germination of the mycoparasite. Aqueous extract of conidia of Oidium euonymi-japonicae was also stimulatory to the mycoparasite. Germination of conidia of A. quisqualis on water agarose decreased as conidial concentration increased. Germination of the mycoparasite's conidia at a low concentration on water agarose was greatly decreased when they were separated by a polycarbonate membrane from conidia of the same species at a high concentration and incubated. After being incubated with conidia of the mycoparasite at a high concentration, water agarose became inhibitory to germination of conidia of the same species at a low concentration, suggesting the presence of a diffusible self-inhibitor. Water agarose is essentially free from nutrients which may support growth of contaminants that interfere with the effect of stimulators or inhibitors.