Rates of production were measured at 22 stations located in a 29-mile segment of the Patuxent River Estuary. The river was sampled in mid-stream at 12 stations which were distributed from a point near solomons, Maryland, to Milltown Landing upstream. Thirteen of the stations were arranged in three transects, which spanned the river in the vicinity of Chalk Point. Standing crops of chlorophyll and rates of photosynthesis were measured at a depth of 1.0 meter on 14 cruises spaced over a 23-month, interval. Concentrations of chlorophyll, and coefficients of carbon assimilation and light extinction were employed to convert estimates at a depth of 1.0 meter to rates of production for the entire water column. Both salinity and thickness of the cuphotic layer showed a considerable gradient in the study area. Salinity increased threefold and thickness of the euphotic zone increased fivefold at the downstream end. The gradient was steepest in the study area upstream of the bridge at Benedict, Maryland. Concentrations of chlorophyll were related to season and position in the estuary. They ranged from 33.3 mg/m3 in October to 3.7 mg/m3 in December, the amplitude being largest at the upstream end of the study area. Average concentration at Station 1 was only 1/3 of the concentration 20 miles upstream at Station 22. The gradient was steeper upstream from the bridge at Benedict. Chlorophyll was least uniformly distributed, in the water column immediately downstream from the bridge. Photosynthetic activity of chlorophyll, at probably subsaturation light intensity, ranged from a mean of 5.21 in August to 1.09 in December. The average for all cruises, when corrected rected to saturation intensities of light, was near a widely accepted value for rates of carbon assimilation per unit of chlorophyll. Rates of carbon assimilation ranged from 2.3 to I48.5 mgC/m3/hr. The months from June through October were the most productive in 1963. However, the Spring of 1964 was con. siderably more productive than was the preceding year, suggesting that the productive period is of longer duration in some years. Seasonal trends in productivity were similar at all stations along a 29-mile segment of the river. The downstream end was only 1/3 as productive on a per-unit-volume basis. Despite the larger rates of production at the upstream end of the study area, the region was least productive per unit of area, owing to the shallowness of the euphotic zone. Mean daily rates for the year were 1.2 gC/m2 for the upstream area and 1.8 gC/m2 for the area downstream from the bridge. The limitations of the computations for the highly turbulent upstream area are discussed.