Standard methods of sediment toxicity testing are fairly well accepted; however, as with all else, evolution of these methods is inevitable. We compared a standard ASTM 10-day amphipod toxicity testing method with smaller, 48- and 96-h test methods using very toxic and reference sediments. In addition we compared parallel exposures of single species, either the amphipod Ampelisca abdita or the mysid Americamysis bahia, to multiple species, mysid, and amphipod, cohabiting the same types of chambers. These comparisons were performed for both water-only and sediment-water tests. Results of the comparison of the standard ASTM 10-day amphipod test with the smaller, 48- and 96-h test chambers indicate that survival was high in both test designs using the reference sediment. With toxic sediments, complete mortality occurred in less than 48 h using the smaller experimental chambers and only after 96 h in the larger experimental chambers. We concluded that although time to death is shorter in the smaller, shorter exposure chambers, there was no overall change in mortality for the organisms, and that the smaller chambers were predictive of the results obtained with larger chambers and longer exposures. For multiple species testing in whole sediment exposures there was no change in toxicity to either the amphipod or the mysid when they cohabited the same chamber. In contrast, for water-only exposures, A. bahia demonstrated less sensitivity when cohabiting the same chamber as A. abdita. Therefore, during whole sediment testing we can add A. bahia and A. abdita to the same test chamber without changing the toxicity to either species; however, in our 10-ml water-only exposures, the species should be tested separately.