A system was developed for testing the chemical stimulation and attraction of free-swimming trematode miracidia to agar or starch gel pyramids impregnated with various chemicals. Short-chain fatty acids, some amino acids, and a sialic acid were shown to attract Schistosoma mansoni miracidia, and also to stimulate attachment and attempts to penetrate the agar. The behavior of S. mansoni miracidia in the presence of absence of stimulating chemicals was classified; it was shown that behavior included directed and nondirected responses to chemical stimuli. The solvent action of distilled water, ethyl ether, acetone, and ethyl alcohol removed attracting substances from Australorbis glabratus, a normal snail host for S. mansoni. Subsequent addition of butyric or glutamic acids to snail tissue, from which the attracting substances had been removed by solvents, restored the capacity of the snail tissue to attract and stimulate miracidia to attempt penetration. Quantitative differences in the responses of Schistosoma mansoni, Schistosomatium douthitti, and Fasciola hepatica miracidia to some attracting chemicals were recorded.