We studied population responses to variation in larval density in a salamander (Ambystoma laterale) that breeds in splash pools on exposed rocky shorelines at Isle Royale, Michigan. Population densities and size distribution were sampled in the field with little error because the water is clear and the pools are kept free of sediment and vegetation by waves from winter storms on Lake Superior. We experimentally manipulated salamander densities in 12 natural pools (three replicates each of four densities) and simultaneously monitored salamander populations in 27 control pools on two islands. The experiment demonstrated that increasing density significantly reduced survival and growth, and increased the skew in the size distributions of individuals. Each of these responses was also observed in the undisturbed natural populations. Density effect were probably due to interference because (1) natural variation in the densities of prey taxa did not influence salamander performance in the experiment, (2) salamander density had no effect on prey availability, and (3) tail damage from attacks by conspecifics was widespread and positively correlated with density. We conclude that density dependence is commonplace in salamander larvae, and probably makes an important contribution to regulating the population size of Ambystoma laterale at Isle Royale.