A series of experiments were conducted to determine the effects of maintenance method (fed or starved), stock location, season, mussel size, and rate of acclimation to temperature on the responses (mortality) of zebra mussels in bioassays. Mussels maintained on a diet of crushed Chlorella are more tolerant to Bayer 73 ยฎ and more sensitive to sodium hypochlorite than starved mussels. Variability in LC50s of zebra mussels is high during the first 260 days in the laboratory, after which the resistance of mussels to both hypochlorite and Bayer 73 ยฎ declines with reductions in body condition. Zebra mussels collected during the early summer and late fall are more tolerant to both hypochlorite and Bayer 73 ยฎ. There is significant variation in tolerances to biocides depending on the stock, such that stocks from locations with more degraded water quality have increased tolerances. Acclimating mussels from 4 to 20ยฐC at rates of 2 and 10ยฐC d-~ does not significantly affect tolerance to biocides. In general, LC50s of mussels vary" by only 2-3 ร, suggesting that mussels from any location, any season, and maintained under any maintenance protocol can be used in "range-finding" tests. Comparisons of results among studies requires knowledge of mussel stock, collection season, and laboratory maintenance protocols.