Arctic charSalvelinus alpinusthat were sympatric with lake troutSalvelinus namaycushin three northern Quebec lakes had faster growth rates, lower survival, and shorter longevities and yielded lower catches per unit effort than a population of Arctic char that was sympatric only with threespine sticklebacksGasterosteus aculeatus. In lakes that contained lake trout, Arctic char were not piscivorous, whereas lake trout were. In the absence of lake trout, Arctic char were piscivorous, the rate of consumption of prey fish was positively correlated with length of Arctic char and, from the age of 6 years onward, each age-class of Arctic char exhibited a bimodal length distribution. We concluded that the growth rate of a small proportion of small, slow-growing Arctic char accelerated when the fish became piscivorous. Internal parasite load was significantly greater in Arctic char when the threespine stickleback was the only other fish species in the community; parasite loads were positively correlated with length and negatively correlated with maturity of Arctic char. Beyond the juvenile stages, lake trout in northern Quebec do not appear to be at risk from the introduction of Arctic char. In Arctic char populations that exist only with threespine sticklebacks, the introduction of lake trout should reduce both the density and parasite loads of Arctic char and increase Arctic char growth rates. Lake trout should be experimentally introduced into some lakes that contain Arctic char to determine if this will increase the value of the fishery resource in those lakes.