The trematodes Cotylurus lutzi Basch (Strigeidae) and Schistosoma mansoni Sambon (Schistosomatidae) use the same snail, Biomphalaria glabrata (Say), as the intermediate host. Laboratory-raised snails were exposed experimentally to miracidia of both species of trematodes at different intervals. Snails harboring sporocysts of C. lutzi became superinfected with S. mansoni as readily as did snails exposed only to S. mansoni. Some snails simultaneously exposed to both species acquired double infections, but snails already harboring S. mansoni could not be superinfected with the strigeid sporocysts. Double infection usually results in the complete inhibition of C. lutzi cercariae and eventual release of S. mansoni cercariae only. Sporocysts of C. lutzi show disrupted germinal balls, pyknotic nuclei, and loss of cellular differentiation and mitotic figures. The sporocysts of S. mansoni in double infections appear normal, but their development may be somewhat retarded. There is as yet no clear evidence to account for this indirect interaction between sporocysts. Interactions between larval trematodes of two different species within single snail hosts have been intensively studied in our laboratory in recent years (summary and bibliography in Lie et al., 1968). Publications on experimental work have reported two types of dual infections in snails: (1) with two trematode species, one developing rediae, the other producing only sporocysts; and (2) with two trematode species, both of which develop redial stages. No report has described a third type of experimental infection: two trematode species, both of which develop in sporocysts. This combination is of particular interest because, in the absence of the predatory rediae, a direct antagonism between the species cannot occur. Only in a sporocyst-sporocyst interaction can an indirect antagonism be investigated free of a complicating direct predatory action. MATERIALS AND METHODS Snails used were the albino strain of Biomphalaria glabrata developed at the National Institutes of Health, Bethesda, Maryland, USA, from Received for publication 10 December 1968. * This study was supported in part by the University of California International Center for Medical Research and Training (UC ICMRT, Hooper Foundation, San Francisco Medical Center) with research grants TW 00144 and AI 07054 from the NIAID, NIH, U. S. Public Health Service. t Current address: Institute for Medical Research, Kuala Lumpur, Malaysia. Reprint requests should be addressed to the G. W. Hooper Foundation. which we also obtained the laboratory strain of S. mansoni. These host and parasite strains were the same ones used in previous studies (Lie, 1966, 1967; Lie et al., 1968). Golden hamsters (Mesocricetus auratus auratus Waterhouse) were used as hosts for adult S. mansoni. We obtained miracidia from the blended and washed livers of hamsters killed 7 to 8 weeks after infection. Adults of Cotylurus lutzi Basch were maintained in society finches (Uroloncha striata (Brisson)) as described by Basch (1969). Eggs obtained from the birds' droppings were incubated in distilled water at 28 C in petri dishes for about 3 weeks before hatching. Snails were usually exposed individually to counted numbers of miracidia (5 to 15) in glass shell vials. Sometimes when miracidia of C. lutzi were unobtainable in sufficient numbers, snails were mass-exposed in larger aquaria. Snails were fed red-leaf lettuce (Lactuca sativa) and chalk and maintained at 26 to 28 C. Doubly exposed snails were maintained individually in plastic aquaria containing about 2 liters of water using an air stone without filtration. Singly exposed snails and controls were grouped in 4-liter glass aquaria fitted with an outside filter-aerator. The snails' transparent shells and lack of mantle pigment made it possible to clearly recognize mother and daughter sporocysts of both species by means of a binocular dissecting microscope and strong transmitted light. Therefore cercarial production was not required to identify single or double infections. To test cercarial release, individual snails were placed in 20 ml of aerated water under a 60-watt incandescent lamp for about 2 hr, usually from 9 to 11 AM. We then estimated and recorded for each snail the number of cercariae of each species that had emerged. The two cercarial types are readily distinguished by their size, shape and form of tail, swimming movements, and other behavioral activities. Forced shedding was undertaken only twice a week, as infected snails usually do not