Several million living cercariae are often needed on a daily basis for protocols in experimental schistosomiasis. Such large daily collections are not usually available, but they may be if a few specific conditions are met. Sandt et al. (1965, J. Parasitol. 51: 1012-1013) established an effective system for mass producing snails and cercariae, but they did not give procedural details for harvesting cercariae. My report describes several procedures for snail maintenance which greatly increase daily cercarial harvests. The intraspecific strains of schistosome and snail may differ in ways which have not been realized. Our Schistosoma mansoni strain originated in Puerto Rico. It has been laboratory-maintained since 1945 in Biomphalaria glabrata of mixed origin: susceptible, pigmented snails brought to the laboratory from Puerto Rico in 1944; and susceptible, albino snails resulting from a cross established in the National Institutes of Health NIAID laboratory by Newton (1955, J. Parasitol. 4: 526-528). The resulting parasites and snails are called the Nmri (Naval Medical Research Institute) intraspecific strains. Before increased cercarial production was necessary, infected snails were kept at 26 to 27 C in clear plastic aquaria which were cleaned twice a week. They were provided with lettuce, dried maple leaves, and chalk. Water was dechlorinated by aging for 4 days. Cercariae were collected every other day from snails placed in beakers of water at room temperature under a fluorescent light. Procedural changes tested and found to provide the largest cercarial harvests are as follows: (1) Two groups of infected snails are maintained in order to provide cercariae 4 days a week while harvesting them from each group of snails only twice a week, Monday and Thursday or Tuesday and Friday. Under our conditions, cercariae appear not to emerge until stimulated to do so by environmental changes described subsequently. Without stimuli for emergence, cercariae are thus not lost in the aquaria but are stored up in the snails until harvested. (2) Exposed snails are maintained at a constant temperature of 27 to 28 C. If kept constantly below 26 C, infections either do not develop, develop very slowly, are held at a low level of cercarial development, or are terminated early in the patent period depending on the temparature level (Gordon, Davey, and Peaston, 1934, Ann. Trop. Med. Parasitol. 28: 323-418; Stirewalt, 1954, Exp. Parasitol. 3: 504-516). (3) Throughout patency, exposed snails are housed in the dark in black-walled and blackcovered 3-liter plastic aquaria which, in turn, are kept in a warm dark snail room. The dark, constantly warm environment eliminates light and temperature changes which presumably stimulate cercariae to emerge (Kuntz, 1947, Trans. Am. Microsc. Soc. 66: 37-49), so loss of cercariae into aquaria from continuous, trickling emergence is minimal. (4) Cercariae are collected from 0800 to 1000 hr. Aquaria containing snails with patent infections are handled one at a time to prevent cercarial emergence into the aquaria. Aquaria are moved singly from the warm dark snail room into the 20 to 22 C brightly lighted laboratory, and the snails are transferred immediately to collecting beakers containing dechlorinated tap water. Beakers are held for the collection period in a collecting chamber which is a temperature-controlled oven, lined with light-reflecting paper. It is illuminated with a 20-W circular fluorescent bulb. This is replaced every 2 mo because, as the bulb ages, the light intensity decreases and cercarial harvests are reduced. Water temperatures in the chamber are from 31 to 33 C. The