We evaluate whether the abundance of the freshwater crabs Aegla itacolomiensis and A. platensis is related to any or all of 11 environmental variables. We sampled 205 Surber samples (33 × 33 cm) in a stream five meters wide in southern Brazil. For each Surber sample we obtained measures of flow velocity, depth, type of substrate, and availability of coarse particulate organic matter. The relationships between abundances of the two species and the environmental variables were assessed by regression trees. Only 2 of the 11 environmental variables were important in describing the abundances of the two species, both of them related to availability of coarse particulate organic matter. The abundance of Aegla itacolomiensis was related positively to quantity of fragmented leaves and, to a lesser degree, to quantity of twigs. For the abundance of A. platensis, quantity of twigs, followed by fragmented leaves, were the most important environmental factors. The quantity of recently fallen, unfragmented leaves was unimportant. We conclude that the two species of Aegla select locations with abundant old plant fragments that are usually colonized by fungi and bacteria. This conclusion is corroborated by previous studies indicating that Aegla spp. feed mostly on plant fragments.