Although reptiles have traditionally been viewed as asocial, the recent discovery of complex social systems in lizards suggests that an animal's social behaviour may be shaped by its interactions with conspecifics early in life, as occurs in endothermic vertebrates. We reared hatchling veiled chameleons, Chamaeleo calyptratus, either in isolation or in groups of four, using a split-clutch design. Social interactions during the first 2 months of life substantially affected a chameleon's subsequent responses to newly encountered conspecifics in standardized trials: animals reared in isolation were more submissive, and adopted darker and duller colours. Isolation-reared lizards also performed less well in a foraging task. Thus, social isolation early in life can impair the development of squamate reptiles, as it does in mammals and birds.