Acute laboratory sediment toxicity tests using the water flea Daphnia magna and the burrowing mayfly nymph Hexagenia limbata were conducted on sediments from two areas of the Keweenaw Waterway, Michigan, to determine whether the tests reflected the condition of benthic macroinvertebrate communities. The organisms were tested simultaneously in a recirculating bioassay chamber in which Daphnia were tested during the first two and last two days of a 10-d Hegagenia test. Sediment from the northern, copper-impacted portion of the waterway was generally acutely toxic to Daphnia, but not to Hexagenia, while sediment from the southern, less impacted portion was not toxic to either organism. Positive relationships existed between organism mortality, copper content of the sediment and field distribution of macroinvertebrates.