The concept of effective food concentration (EFC), a means of predicting food consumption from selectivity and food concentration data, is explained, tested, and applied to understanding food consumption by the freshwater copepod Diaptomus sicilis on mixtures of algae of different sizes and on Lake Michigan seston. Experiments on mixtures of different sized Chlamydomonas spp. showed that selection (W′) was an invariant function of particle size when the algae were counted microscopically. When the Coulter counter was used, a more variable pattern of selectivity — similar to the peak tracking response reported by some investigators - was obtained. This was due to bias of zooplankton-produced particles. Size-selective selectivity coefficients (W') were used to weight the food concentration in each size category and the weighted values summed to give EFC. Food consumption in experiments with seston and with cultured algae was better described by EFC than by total food concentration (TFC), the unweighted sum. Moreover, use of EFC diminished the magnitude of the apparent threshold concentration required for feeding to commence. Although selectivity in algal mixtures and lake seston was approximately the same, the food consumption versus EFC curve saturated more quickly for the algal mixtures than for the lake seston. Since expression of food concentration as EFC allowed direct comparison of experiments having different particle-size spectra of food, we concluded the difference resulted from the lower food quality of lake seston, that is, its lower digestibility and sensory quality for zooplankton capture.