Newly metamorphosed anuran amphibians of several species have been observed to form very dense aggregations around the peripheries of breeding ponds; in some cases, the animals will literally pile on conspecifics. Two hypotheses used to explain the behavior were tested in several experimental situations: that juvenile anurans aggregate as a response to (i) predation risk or (ii) desiccation. Two experimental setups were used to test the first hypothesis. There was no tendency for nonphysiologically stressed juvenile Bufo americanus to group on a relatively high-risk substrate or in the presence of a preformed group, with or without a predator present. The results of two additional experiments showed that these animals aggregated as a response to desiccation and that animals desiccated alone suffered greater weight loss and mortality than those desiccated in groups. Thus, the results strongly support the hypothesis that these young anurans aggregate in response to the risk of desiccation, not to the risk of predation.