The filtration and ingestion rates of Diaptomus oregonensis females and stage V copepodids were examined in three experimental situations. When unialgal cultures of 20,000 cells per milliliter were offered to D. oregonensis the filtration rates remained constant for cells ranging from 120 to 10,000 µ3. When a mixture of diatoms was offered the animals did not filter cells ranging from 1 to 60 µ3 despite the fact that the cells in this size range were concentrated at about 20,000 per milliliter. When natural phytoplankton from Marion Lake was used as a food source, the filtration rates were low for cells smaller than 100 µ3, increased to a maximum of 12.9 ml per animal per day for cells ranging from 102 to 333 µ3, and remained constant with increased cell volume and decreased cell concentration. Several species of Cyclotella and Merismopedia were not removed from the Marion Lake plankton by D. oregonensis.