We developed a model predicting the filtering efficiency of different sizes of filter-feeding gizzard shad Dorosoma cepedianum. The model is based on the cumulative frequencies of interraker distances of the gill rakers and was corroborated by feeding trials in which gizzard shad ingested different sizes of suspended plastic microspheres and plankton. The proportion of particles removed by fish increased as a function of particle size, leveling off when particle diameters exceeded 35 μm for small fish (7.1–11.1 cm standard length, SL) and 55 μm for large fish (13.6–16.3 cm SL). According to selectivity index values computed from the filtering efficiency model, gizzard shad of 5, 15, and 25 cm SL would selectively feed on particles larger than 19, 40, and 63 μm, respectively. This change in selective particle ingestion may explain why phytoplankton becomes less important in the diet of gizzard shad and why this clupeid shifts its feeding niche as it increases in length.