Under described experimental conditions, miracidia of Schistosoma mansoni show moderate, but definite, attraction toward Australorbis glabratus, but not Helisoma anceps or Bulinus spp. Statistical evaluation of these data indicates a high degree of confidence may be placed in the results but should not be taken to indicate either the strength of the chemical stimulus or response involved or show the importance of such chemical perception in the location of hosts under natural conditions. There is no doubt that both light and gravity are far more powerful stimuli in determining the orientation of S. mansoni miracidia than the chemical ones produced by their molluscan host. Controversy exists concerning chemotactic responses of trematode miracidia to their molluscan hosts and their secretions. Observations of Malek (1950) and Campbell and Todd (1955) indicate no specific attraction, while those of Faust (1924) and Wright (1959) support the view that miracidia perceive and are attracted by mucus secreted by their snail hosts. There are few published accounts which are not subjective in describing this phenomenon, though the study of Kloetzel (1958) made an attempt to quantitate subjective data. The present study was undertaken to obtain more objective data and to subject the result to statistical evaluation. MATERIALS AND METHODS A cast-iron center chamber, 4 cm high x 8 cm diameter, was constructed with four side arms, 3.5 cm long X 1 cm diameter, which connected to four vertical terminal chambers, 3 cm high x 1.5 c diameter. The entire interior surface was coated with inert waterproof lacquer to prevent contamination and to permit thorough cleaning of the chamber. One snail was then placed in each of two of the terminal chambers, the entire apparatus filled with conditioned water, and allowed to stand for 1 hr or more to allow substances produced by the snails to enter the water. The shells of snails were routinely cracked to prevent their moving into the center chamber. About 100 to 200 miracidia were placed in the center chamber, which was then covered, and the entire apparatus placed under a strong light to prevent miracidia which had reached a terminal chamber from returning to the center chamber. Miracidia were obtained by the method of Lee and Lewert (1956) from the livers of mice inf cted with Schistosoma mansoni (PR Strain). Sham snails of inert aquarium cement were molded to the size and shape of the snail being used and placed in the other two vertical chambers to equalize the amount of light reflected into the center chamber through each arm. After 1 hr, 60 to 80% of the miracidia had entered the side arms, which were then stoppered at the end nearest the center chamber. The contents of each arm and conjoined terminal chamber were then drained and miracidia counted separately. The entire chamber was cleaned carefully and dried between experiments in order to prevent even minute accumulations of snail mucus from influencing subsequent experiments. The chambers into which snails were placed were varied randomly from one experiment to another for similar reasons.