With increased use of toxicity testing for monitoring effluent toxicity, it is important to evaluate the precision of effluent toxicity tests To address this need, we identified, compiled, summarized, and evaluated published and unpublished data from intralaboratory and interlaboratory studies on the variability of acute and chronic effluent toxicity tests for single chemicals and effluents A total of 23 published and unpublished reports were identified The most extensive data on test variability are available for acute tests with Daphnia spp, Pimephales promelas, and Mysidopsis bahia and chronic tests with Pimephales promelas, Ceriodaphnia dubia, and Cyprinodon variegatus All other tests have considerably fewer data Data on which to evaluate the variability of toxicity tests produced by the most current U S Environmental Protection Agency methods are not extensive Mean C V s for intralaboratory studies (7‐33%) were smaller than those for interlaboratory studies (34‐46%) However, mean intra and interlaboratory C V s for chronic tests (7‐38%) were generally equal to or smaller than those from acute tests (17‐46%) Also, C V s from single chemical tests (32‐46%) were generally larger than those from effluent tests (7‐34%)