Abstract Optimal conditions for collecting in vitro large and reproducible harvests of schistosomules are described. They consist of: (1) penetration membranes of excised abdominal skin of female rats about 60 days old prepared by rubbing-drying-sanding-plucking; (2) a single application to each skin membrane (1500 mm 2) as soon as possible after their emergence, of about 5000 cercariae in an aqueous suspension about 5 mm deep; (3) a darkened cercarial suspension at a temperature of about 28 °C over a brightly lighted collecting medium of Hanks' balanced salt solution with penicillin and streptomycin at a temperature of 37 °C; (4) collection of schistosomules for about 3 hours. Schistosomules must be differentiated from postpenetration cercariae in the harvests. Schistosomules lack tails, acetabular gland contents and precise surfaces, and are intolerant of water. Schistosomules collected in vitro matured in mice similarly to those produced by cercarial penetration of skin in situ. Four factors were highly influential upon the schistosomule harvests as tested quantitatively and qualitatively: type of membrane, number of cercariae applied, and the composition and temperature of the collecting medium. Two factors studied did not affect the schistosomule harvest: sex of the 60-day-old rat skin donors, and a uniformly light vs. a uniformly dark environment. The seven other factors were moderately influential. Cercariae displayed sensory perception of certain stimuli by responding to them with movement toward their sources. These stimulus sources were: host skin, but not a Baudruche membrane; strong light in an environment with a light differential; 37 °C temperature of the collecting medium when the cercarial suspension was 23–28 °; some component(s) which diffused through the skin membrane from the collecting medium of HBSS.