The bottom waters of the mesohaline portion of Chesapeake Bay become depleted in oxygen in summer. We found that copepods and nauplii were in low abundance or absent from bottom waters when oxygen concentrations were < 1 mg 0, litcr-I. In contrast, when oxygen concentrations were higher in bottom waters in spring or summer due to episodic mixing events, the highest copepod abundances were often found in bottom waters. Laboratory experiments confirmed that oxygen concentrations < 1 mg 0, liter-1 resulted in reduced survival of the copepods Acartia tonsa and Oithona colcarva and inhibited the hatching of A. tonsa eggs. The decrease in Chesapeake Bay copepods in May-June parallels the decline of oxygen in bottom waters. Our field and laboratory data suggest that this dcclinc in copepods could result from reduced recruitment as a consequence of egg mortality in the low-oxygen bottom waters. In summer this source of mortality would be reduced because warmer water temperatures would allow the eggs to hatch in the upper water column above the low-oxygen bottom waters.