Interference and resource competition by adults inhibited growth rates of conspecific juveniles of the land snail species Mesodon thyroidus and Neohelix albolabris in separate field and laboratory experiments, but not in laboratory experiments on Anguispira alternata. In 1 m2 field cages at near-natural densities under ambient food and water conditions, juvenile M. thyroidus apparently competed with adults for food or water or both resources, growing more slowly when living with two conspecific adults, but being unaffected by adult presence when food and water were augmented. Neohelix albolabris juveniles were similarly unaffected in field cages by presence of two adults when food and water were augmented. In contrast, interference, not resource competition, apparently explained growth inhibition in laboratory cages at densities considerably greater than natural densities, with non-limiting food and moisture; both M. thyroidus and N. albolabris juveniles grew more slowly as conspecific adult number increased from zero to three.