Feeding selectivities of Daphnia galeata were measured using various density combinations of 14C-labeled Chlamydomonas and 3H-labeled natural bacteria. In all food combinations, D. galeata preferred algae. Selectivity coefficient (bacterial clearance rate: algal clearance rate) was 0.35 at the level of incipient food limitation (1.5 106 m3 ml–1).However, it increased up to 0.9 with increasing total food concentration. A simple mechanical sieving model alone cannot explain such a change in the selectivity coefficient. A mechanism of clogging suggestively explain the increase of bacterial ingestion efficiency by D. galeata, especially at high food concentrations. The change of selectivity coefficient related to food condition suggests that bacterial feeding by the Daphnia is incidental event associated with the algal feeding.