changes in light and temperature from the dark 27 to 28 C snail room to the bright 31 to 33 C collecting chamber stimulate developed cercariae to emerge abundantly. (5) If desired, the cercarial harvest can be increased even more by a double shedding procedure in which emerged cercariae are poured off the snails at 0900 hr, the beakers refilled with 22 to 23 C water, and cercariae collected again at 1000 hr. (6) Individual snails 5 to 7 mm in diameter are exposed to miracidia to insure equal exposure. Using a fine micropipette, 8 to 10 miracidia are transferred in a small drop of water into the center of a flat-bottomed glass vial (15 x 60 mm) which has been siliconed to prevent spread of the drop. Each tube is checked to insure the correct number of miracidia. Five milliliters of water and one snail are added and the vials are kept in the snail warm room overnight. Snails are then maintained as described above. (7) Insofar as possible, the growth of rotifers which colonize the snail shells, immobilizing the cercariae, decreasing the numbers which emerge and reducing their infectivity (Stirewalt and Lewis, Int. J. Parasitol., in press) is controlled by swabbing the shells with 10% ethanol. Especial care is taken to clean the shell crevices. The alcohol-treated snails are rinsed in dechlorinated tap water before they are put in clean aquaria. Using these procedures, we have for several years been collecting an average of 2,800 (range from day to day of 2,000 to 4,000, occasionally 5,000) cercariae per snail per day. Under the usual laboratory conditions of uncontrolled light and temperature and cercarial harvesting every other day, our average was 1,000 (range of 600 to 1,800) cercariae per snail per day. These averages were calculated from the total cercarial harvest from snails in all stages of patency of infection. These conditons have not been thoroughly tested on any other intraspecific associations of Schistosoma mansoni and Biomphalaria glabrata. Some modifications may be necessary for optimal cercarial production from other parasite-snail combinations. This work was supported by an Office of Naval Research Contract No. N00014-76-C0146. e is, Int. J. Parasitol., in press) is