The diet of the opossum shrimp, Neomysis mercedis, in the Sacramento River Estuary was studied in relation to food availability, i.e., plankton, from January through November, 1976. The composition of the diet of N. mercedis varied in relation to mysid size and prey availability. Mysids exhibited strong positive selection for the large diatom prey species while "avoiding" small diatom prey. Although diatoms were the most abundant prey identified from the guts of specimens of N. mercedis it was determined that predation on rotifers and copepods accounted for \> 80\% of the energy consumed by other-than-juvenile mysids (>=7 mm in length). Juvenile mysids (<= 3 mm in length) ingested rotifers when rotifers were abundant but were not found to consume copepods. Laboratory feeding experiments indicate a density-dependent feeding by N. mercedis on copepods, i.e., as copepod density increases mysid predation on copepods also increases. Feeding observations indicate that N. mercedis is not a particularly active predator, capturing prey drawn into its feeding current but not actively pursuing prey. N. mercedis appears to feed continuously, with a peak in activity for mature mysids during darkness, a pattern not apparent in immature mysids. Consumption of the detritus was not considered significant. Although herbivory may be of direct importance during the spring diatom increase, carnivory was the major source of energy for N. mercedis in the Sacramento River during 1976.