Diet and growth were compared for lake whitefishCoregonus clupeaformiscaught before and after the recent introduction ofMysis relictato Flathead Lake, Montana. Most foods and seasonal feeding patterns were similar, except that lake whitefish now also eat mysids. Mysids were eaten by fish larger than 100 mm total length, and especially by fish larger than 400 mm collected in the fall. Mean length at first annulus was less in most years following the mysid introduction than before (Student'st,P< 0.05). Mean lengths at other annuli were similar between the two periods. Larger lake whitefish may be less sensitive to reduced zooplankton abundance caused by mysid predation because they feed on benthic fauna. They may also benefit from the recent availability of mysids for their diet.