The Susquehanna River annually supplies about 8.4 × 104 and 4.7 × 103 metric tons of particulate carbon (PC) and nitrogen (PN), respectively, to upper Chesapeake Bay. In the upper bay, the concentration of PN usually ranges between 0.10 and 0.30 mg liter−1 and is occasionally greater than 0.50 mg liter−1. In the lower study area, the concentration of PN stabilizes near 0.10 mg liter−1. Maximum values of the carbon:nitrogen (C:N) ratio (atomic basis) occurred in the upper bay, and highest values were associated with the late-winter thaw period of the Susquehanna River. C:N ratios of 20–30 were usual most of the year in the low salinity region, and often greater than 30 during times of maximum river discharge. In the lower study area, the ratio approached 15 throughout most of the year. In general, the reduction in the C:N ratios of the suspended material was reflected in the C:N ratios of the sediments.The high C:N ratios in the upper bay and the tidal freshwater portion of the Susquehanna River indicate a high detrital content. An estimate of net primary production would extrapolate to the fixation of PN of approximately 1600 metric tons per year in the upper bay or about 34% of that supplied by upland drainage. The major source of PN in the lower study area is probably provided by photosynthetic fixation, since the physical circulation of the estuary retards a large movement of particulate material seaward.