Grazing on natural particulate assemblages by the copepods Acartia tonsa Dana, Centropages velificatus De Oliveira and Eucalanus pileatus Giesbrecht was examined in shipboard experiments in the Mississippi River plume (northern Gulf of Mexico continental shelf)- Copepods fed during simultaneous experiments in food-rich surface waters of the plume and in the less-food-rich waters 8–12 m below the plume. Grazing by these three copepods was generally nonselective. Ingestion and filtration rates, as well as electivity of individual phytoplankton taxa, were calculated after phytoplankton cell counts were completed from 51 experiments. Phytoplankton species were eaten primarily in proportion to their abundance, without regard to differences in cell size and/or shape. Plots of electivity vs. two different measurements of cell size (cell volume and maximum linear dimension) for each of the 51 individual experiments revealed only three cases of selective feeding. These quantitative results support previous qualitative studies of fecal pellet contents using scanning electron microscopy.