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Environmentally-driven paedomorphosis, the retention of juvenile characteristics by adults, is an overlooked alternative to adaptive genetic hypotheses regarding shell colour in landsnails. We previously considered two adaptive explanations of shell-colour polymorphism in the desert landsnail Trochoidea simulata, namely temperature and predation. However, neither of these factors adequately explained this colour variation. We subsequently described the importance of available CaCO3 as the driving force underlying colour variation in this species. Here we consider the consequences of differential access to CaCO3 by individuals of different colour morphs. We describe variation in shell shape of different colour morphs, and conclude that shell-colour variation in this species is a consequence of paedomorphosis in brown individuals. Individuals may change colour ontogenetically from brown to white, while some remain brown-shelled throughout their lives. Brown snails, like juveniles, are thin- and flat-shelled, and have relatively larger apertures than white snails. This paedomorphosis is environmentally-based, being correlated with soil CaCO3 content. (C) 1997 Academic Press Limited.