Fifty of each sex of two species of terrestrial isopods, Armadillidium vulgare and Porcellio laevis, were each offered 16 live pupae, four each of Drosophilia hydei, D. immigrans, D. pseudoobscura and D. melanogaster, for 24 hr. Both sexes of both predator species fed on all prey species, each predator species taking numbers of prey species in the reverse order of prey size, namely, D. melanogaster > D. pseudoobscura > D. immigrans > D. hydei. The results, however,do not indicate preference by predators. In terms of total weights consumed, Porcellio ♂ ♂ and ♀ ♀ ate 8.91 and 8.04, and Armadillidium ♂ ♂ and ♀ ♀ 11.69 and 11.86 mg/isopod°day respectively. The difference between sexes is insignificant, but between predators it is highly significant (P < 0.001). Ratesof consumption of Drosophila pupae and of Hippelates pupae decreased by about two—thirds or more over a week in the absence of other food. Laboratory experiments using known numbers of 32P—labelled Drosophila pupae fed to Armadillidium proved the feasibility of using the technique for field studies. Such work subsequently showed that A. vulgare ate D. melanogaster pupaein natural conditions in a citrus grove where alternative food was abundant. If predation by isopods in nature proves to be wide—spread, their significance in several ecosystems would deserve reconsideration.