In land snails, intraspecific crowding has been shown to affect growth rate, adult size, fecundity, survival, and activity, all of which can influence subsequent population growth. Mechanisms of crowding effects included resource limitation, and behavioral or chemical interference. I examined the effects of adult presence on growth rates of conspecific juveniles and the mechanisms of those effects in field and laboratory studies for three species of land snails, Mesodon thyroidus, Neohelix albolabris, and Anguispira alternata. I also developed a spool and line technique to study movement patterns in the field. In field cages, juvenile M. thyroidus grew faster alone than with two conspecific adults. However, when food and water were non-limiting, growth rates of M. thyroidus and N. albolabris juveniles were unaffected by conspecific adult presence. In laboratory cages with abundant food and moisture, both M. thyroidus and N. albolabris juveniles grew more slowly as conspecific adult number increased from zero to three, indicating that interference, not resource limitation, explained growth inhibition. However, growth of juvenile A. alternata was unaffected by conspecific adults. For N. albolabris, growth was inhibited by either contact with or odors of total conspecific adult products, but not by feces or mucus separately, suggesting that allelochemicals acted synergistically and were volatile. For M. thyroidus, responses varied. In two experiments, feces or mucus alone reduced growth. In three other experiments, neither total products, feces, nor mucus affected growth. Behavioral inhibition by adults may also occur in M. thyroidus. Although allelochemical interference can inhibit growth in laboratory cages, resource limitation may be more important in limiting growth in nature.