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Homotypically conditioned media (Hom. C.M.) produced by adult Biomphalaria glabrata weighing about 600 mg and fed on pure cellulose under bacteriostatic conditions at a density of one snail per 100 milliliter of standard snail water for three days significantly enhanced the growth of juveniles. Hom. C.M. produced under the same conditions by fasting snails that were previously fed on lettuce, also had a growth promoting effect but was less potent. The growth promoting factors have a molecular weight of 500-104, and appear to act directly on the snails and not indirectly by influencing the food medium. The extent to which absolute growth rate is stimulated by the growth factors correlates well with decrements in the conductivity and calcium concentration of the media. Possible modes of action of the growth factors are discussed. They appear not to influence the relative weights of shell, or body and water content of the snails. It is suggested that the adaptive significance of the growth factors is to facilitate rapid increase in numbers of this gregarious and often fugitive species when environmental conditions are favorable.