Pythium aphanidermatum zoospores in distilled water suspension showed differential responses towards a range of single compounds, and mixtures, diffusing from the ends of capillary tubes containing these materials in agar. All substances caused an initial disorientation of zoospore movement followed, according to the substance under test, by indifference, repulsion, attraction, trapping, and encystment, or, at extreme pH, by loss of motility and death. Of a wide range of substances and mixtures tested, including sugars, amino acids, inorganic salts, organic acids, auxins, and vitamins, only glutamic acid, after weak base adjustment, and mixtures of sugars combined with mixtures of amino acids, induced the pattern of response observed with roots and root materials. Comparison of the activity of glutamate and structurally related compounds indicated that the effectiveness of glutamate was dependent on the presence of several moieties of the glutamate ion.Short photographic exposures allowed counts to be made of zoospores and cysts around the capillary mouth and provided a method for comparing, quantitatively, the accumulation of zoospores in response to different substances over a period of time.The substances inducing activity were readily detected in pea root exudate and extract and are, in fact, of general occurrence in roots. The effectiveness of the active principle(s) in root exudate and extract was roughly proportional to the concentration of these root materials in terms of their content of carbon and nitrogen but no such relation could be established for glutamic acid and a joint mixture of amino acids and sugars, suggesting the existence in root materials of additional factor(s).