The hard clam (Meretrix lusoria) is one of the most important cultured molluscs in Taiwan. Although it is mainly cultured in ponds and supplemental feeding of artificial feed is commonly practiced, no scientific information on its preferred foods and appropriate feeding strategies are available. Six diets: fishmeal (F), soybean meal (S), commercial hard clam feed (C), microalgae Tetraselmis chui (T) and Nannochloropsis oculata (N) and bread yeast (Y), were fed to hard clams. Each diet was fed at 3 concentrations (20 mg L−1, 200 mg L−1 and 633 mg L−1) and was fed to clams of 3 sizes (7–11g, 12–17 g and 18–26 g total body weight) to gauge their effects on weight-specific filtration rate and pseudofeces production. Overall, the filtration rates for diets in descending order were S > C > F = Y > N > T. Particle size of the diets played an important, but not the only. role in determining the filtration rate. Diets with regular particle shape (N, T, Y) yield lower filtration rates and higher pseudofeces production than diets with irregular shape (C, F, S). Both filtration rate and pseudofeces production were higher for yeast than for algae (T, N). Filtration rate and pseudofeces production were also higher for the mixed component diet (C) than for single component diets (F, S). Filtration rate, but not pseudofeces production, was higher for the plant-based diet (S) than for fishmeal (F). Both diet concentration and clam size had inverse correlations with weight-specific filtration rate. The effects of all interactions between diets, diet concentrations, and clam sizes on filtration rates were significant, reflecting the complexity of feeding behavior of Meretrix lusoria.