The size of the zooplankton consumed by rainbow trout and yellow perch was studied in two Michigan lakes and compared with the size of the limnetic zooplankton. One of the study lakes was managed for both rainbow trout and warmwater game fish (a “combination” lake), and the other was managed solely for rainbow trout after it was reclaimed with toxaphene. Plankton samples were collected in the summer both before and after the introduction of rainbow trout, and compared with plankton in the stomachs of fish taken at the same time.