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Fitchburg, Massachusetts sanitary landfill leachate was subjected to toxicity tests using: fathead minnows (Pimephales promelas), zooplankton (Daphnia magna), green algae (Selenastrum capricornutum) and aerobic luminescent bacteria (Photobacterium phosphorium). The leachate was highly toxic to the test bacteria, moderately toxic to daphnids, and slightly toxic to fathead minnows. Algal cells, unable to grow at the 10-percent leachate exposure level, recovered after centrifugation and reinnocuation into algal nutrient medium. Low-flow summer hydrological data indicated that the leachate contributed about 7% to the total flow of the receiving stream, Flagg Brook, and about 0.6% to Sawmill Pond water located further downstream from the leachate outfall. These data, together with observed toxicity values for the test organisms, indicate that the leachate concentration in Flagg Brook impacts the diversity of aquatic life in this system, but may be less severe in Sawmill Pond where increased dilution results in leachate levels below the acutely toxic level. The considerable variation between toxicity test results obtained with the four test organisms, demonstrates the importance of conducting several such toxicity tests using organisms from different trophic levels, to assess the potential impact of a pollutant discharge on an aquatic ecosystem.