Life-cycle tests on the survival and reproduction of the amphipods Gammarus lacustris and G. pseudolimnaeus were conducted under two nontoxicant experimental conditions: with several amphipods per tank and with individual pairs per tank. Juvenile and adult survival of the two species was similar with several amphipods per tank, but was higher for G. lacustris among the reproducing individual pairs. In the multiple-animal tanks G. lacustris females had larger broods than did G. pseudolimnaeus females and both species produced more young per female and more broods per female than in the tanks with individual pairs. Our results indicate that these amphipods may be reared more successfully with several animals per tank than with only two. Of the two species, G. lacustris may be preferred for use in chronic toxicity tests because of its greater fecundity, larger size, and wider distribution.