Stomach content analyses were performed on dwarf specimens of Asplanchna sieboldi collected on different dates in a shallow floodplain lake of the Orinoco River system. The results showed that the dwarf forms of A. sieboldi were highly selective on small Brachionids (Brachionus caudatus and Keratella americana). This observation confirmed on the carnivore status of these forms of the species. Considering the aleatory character of prey capture in rotifers, the high selectivity observed in A. sieboldi may be controlled by the abundance of the favoured small-sized prey. Under the conditions of the study, Asplanchna may be considered a stenophagous predator. The prédation pattern exhibited by the dwarf A. sieboldi under our tropical conditions was similar to the pattern exhibited by larger species of Asplanchna in temperate regions. In both regions, size and abundance of prey seem to control the food preferences of this planktonic predator.