Selectivity coefficients (W′) and predation rates on Lake Michigan zooplankton were determined forMysis relicta during spring through fall using anin situ method. W′ values indicated the following ranked order of prey preference: Cladocera > copepod copepodites and copepod nauplii > adult diaptomids and cyclopoids. With few exceptions, W′ values for different prey categories remained fairly constant despite greatly changing relative abundances of prey. Predation rates and prey selectivity were similar in most cases to those determined in laboratory studies. Ingestion rates (percent dry body weight · day−1) were correlated to total prey biomass (r = 0.38) and to effective prey biomass (r = 0.85), where the weighting factors were overall mean selectivity coefficients for the different prey categories. This result suggested that seasonally varying composition of prey caused much of the variation in ingestion rates among experiments. Feeding trials performed at the same depth with daytime and nighttime assemblages of zooplankton indicated that Cladocera may escape heavyMysis predation at night by migrating from the metalimnetic-hypolimnetic interface into the epilimnion.