AbstractThe toxicity of 12 freshwater sediments to the water flea,Daphnia magnaStraus, and the mayfly nymph,Hexagenia, were tested in recycling laboratory microcosms. The organisms were tested both together and singly. Significant differences inDaphniamortality between test and control sediments occurred in only two cases in whichDaphniawas tested alone, and in five cases in which it was tested withHexagenia.SignificantHexageniamortality occurred in only two cases. The results imply that (a)Hexageniais a less sensitive indicator of freshwater sediment toxicity than isDaphniaand (b) the presence ofHexageniausually intensifies theDaphniaresponse. Tests that combine these two organisms are recommended, sinceDaphnia, although an open‐water species, responds not only to dissolved materials in the water but also to particulates released from the sediments by the physical activity ofHexagenia.These types of tests could be used, on a standardized, routine basis, as indicators of potential problem