Although the alewife (Alosa pseudoharengus) has been the principal prey fish of adult lake trout (Salvelinus namaycush) in Lake Ontario, a lakewide survey of PCB concentration in Lake Ontario alewife was not attempted until 1986. We applied an individual-based model to the 1986 population of Lake Ontario lake trout to (a) estimate the PCB concentration in alewife prior to 1986 and (b) determine the amount of variability in PCB concentrations of lake trout prey. Modeling results indicated that the PCB concentration in adult alewife during 1979-1985 was on the average almost four times greater than the adult alewife PCB concentration observed in 1986. This result suggested that PCB concentration in lake trout prey may fluctuate widely from year to year, perhaps in response to changes in age structure and growth of prey fishes. Secondly, modeling results suggested that spatial variability in the PCB concentrations of lake trout prey was probably greater in Lake Ontario than in Lake Michigan.