Abstract.-Stomach contents were examined from 1,164 yellow perch (Perca flavescens) collected from Little Bay de Noc and Big Bay de Noc, Lake Michigan, June through October, 1988. Diet through August consisted of several different taxa considered typical for yellow perch. During September and October, 124 perch (76 to 213 mm total length) contained 1-120 (mean = 16.7) of the exotic cladoceran Bythotrephes cederstroemi. Of 39 other fish species examined, only rock bass (Ambloplites rupestris) was found to have ingested Bythotrephes. Trout-perch (Percopsis omiscomaycus) had considerable dietary overlap with yellow perch, but no trout-perch was found to have eaten B. cederstroemi. The overall proportion of yellow perch preying on B. cederstroemi was higher in Little Bay de Noc (32%) than in Big Bay de Noc (4%). Within individual fall net samples, from 0 to 72% of the yellow perch contained Bythotrephes. Yellow perch appeared to prefer B. cederstroemi (when available) over other food items that had comprised their diet earlier in the year and continued to be available during the fall. Many age-0 yellow perch were large enough by September to consume Bythotrephes, but 1- to 4-year-old fish may have been more efficient in handling the cladoceran. If B. cederstroemi become abundant prior to September in subsequent years, age-0 perch may not be of sufficient size to prey on them.