Continuous-flow and static bioassays were conducted at 18 C, with survival and reproduction as measures of relative toxicity of eight PCB's, Aroclor 1221 (A-1221), A-1232, A-1242, A-1248, A-1254, A-1260, A-1262, and A-1268. Three PCB-mixture bioassays were also conducted. Aroclor 1248 was the most toxic to Daphnia magna of the eight Aroclors tested in static tests; the 3-wk LC50 was 25 μg/liter. Aroclor 1254 was the most toxic PCB to Daphnia under continuous-flow conditions with a 3-wk LC50 of 1.3 μg/liter. Ninety-six-hr LC50 values for A-1242, and A-1248 on Gammarus pseudolimnaeus in continuous-flow tests were 73 and 20 μg/liter. Survival after 60 days was 52% at 8.7 μg/liter 1242 and 53% at 5.1 μg/liter A-1248. Reproduction and survival of young were good at 2.8 μg/liter A-1242 and 2.2 μg/liter A-1248. The midge Tanytarsus dissimilis, in continuous-flow tests, did not emerge in abundance above 5.1 μg/liter A-1248 or 3.5 μg/liter A-1254. The 3-wk LC50 for Aroclor 1254 was 0.65 μg/liter for larvae and 0.45 μg/liter for pupae. Tissue residues in Gammarus pseudolimnaeus ranged from 4.0 μg/g A-1254 in control animals to 552 μg/g A-1248 in scuds held for 60 days in water containing 5.1 μg/liter A-1248.