At 138 stations in the eastern Mississippi Delta area measurement were made of phytoplankton production (C¹⁴O₂ uptake), chlorinity, temperature, suspended solids, Secchi disc depth, inorganic phosphate, soluble silica, and soluble Kjeldahl nitrogen. In addition, the phytoplankton in the water were identified and enumerated. These stations were generally located inshore of those taken by G. A. Riley (1937). The following results were obtained.

1. Surface phytoplankton production off the delta is equal to or greater than that of highly productive tropical or subtropical pelagic or neritic areas.

2. Surface production is quite variable; variation can be as much as seven-fold from one day to the next, at a given location.

3. Although seaward traverses made on single days showed that production generally increased at seaward locations, there were no over-all statistical differences between river, plume, and gulf areas at any given season.

4. During the period of high river discharge (May), surface production at the most seaward locations (gulf) was significantly greater than during the fall months. Phosphate and Kjeldahl nitrogen were also significantly greater in May than during the fall months in this area.

5. Integrated production in the water column was three to six times that occurring at the surface. Measured in situ water column production agreed within 20 per cent with that calculated by Ryther's (1956) method in three out of six determinations. In the other determinations, the measured value was three to six times greater than the calculated value.

6. Rough calculations of the rate of sedimentation of organic carbon off the delta were compared with phytoplankton production. This comparison showed that it is unlikely that production contributes much organic carbon to pro-delta slope sediments. Production would have to proceed at the maximum rate throughout the year without losses of phytoplankton carbon to equal the rate at which carbon is deposited in these sediments. In the bottom-set beds farther seaward we cannot estimate from the present data the relative contribution of organic matter by phytoplankton to these sediments.

7. Theoretical calculations indicate that nitrogen is a more likely limiting nutrient in these waters than phosphate. Silicate may limit phytoplankton only in highly saline water, where no silicate could be detected.

8. Two hundred species of phytoplankton were identified from the delta, of which 86 per cent were diatoms. Two associations of primary species were recognized. One of these, consisting of species of Cyclotella, Melosira, and Navicula, was found in the river plume, and Blind Bay. The other consisting of species of Nitzschia, Thalassionema, Thalassiothrix, Skeletonema, Asterionella, and Chaetoceros, was found in the gulf, plume, and Breton Sound. During any given season the relative concentrations of phytoplankton do not differ significantly from area to area. Significantly fewer phytoplankton were found in the river and plume at times of low water, but no significant difference in phytoplankton concentration in the gulf was found between May and the all months.