Zooplankton in the tidal, freshwater Potomac River were usually unstratified in the water column. Vertical mixing caused primarily by tides apparently prevented stratification, hence the classic pattern of diel vertical migration was not found. Larger grazers could occasionally concentrate in deeper waters during daylight hours, so the mean distributions of zooplankton grazing pressure was slightly skewed towards the bottom. Zooplankton tended to exhibit patchy horizontal distributions, and densities along river transects could vary by an order of magnitude. Populations of zooplankton grazers during the 1981 summer could hypothetically remove an average of of 1.1% to 3.7% of the water column's phytoplankton each day. Estimated values ranged as high as 9.7% day-1.