Estimates of annual primary production and standing crops based on chlorophylla were made in the northern half of Chesapeake Bay over a 160 km section. Both the chlorophylla concentrations and incubator productivity were maximum during the warmer season, especially in the oligohaline area. The higher salinity waters usually showed lower values of both parameters, except during the winter when values were fairly uniform over the entire study area. Chlorophylla values ranged from 1 to 80 mg m−3 up-bay at Station I-B, and 1 to 25 mg m−3 down-bay at Station XI. Incubator photosynthesis ranged from 2 to 200 mg C m−3 hr−1 up-bay and 2 to 40 mg C m−3 hr−1 down-bay. Chlorophylla concentrations and incubator productivity were usually low at the tidal freshwater station, rapidly increased in the oligohaline area and tended to decrease with increasing salinity. The seasonal pattern was characterized by one general maximum during the warm months with little tendency for a spring bloom and, occasionally, some stations showed a late fall pulse in standing crop and photosynthesis. Photosynthetic activity per unit chlorophylla at the down-bay stations ranged from 1.3 to 10.3 mg C hr−1 mg Chla−1 and the seasonal average was 3.9. The assimilation numbers at the up-bay stations ranged from −1 (mg Chla−1 and averaged 3.3 and 1.1 during the warm and cold periods, respectively. Computed values of integral gross photosynthesis averaged about 0.2 g C m−2 day−1 at the up-bay stations and approximately 1.0 g C m−2 day−1 at the down-bay stations over the period of November, 1966 to October, 1967. Turbidity of the waters appeared to have a controlling influence on primary production in the water column. However, photosynthetic activity was highest in turbid up-bay water on a volume basis.