1. A dense population of planktonic diatoms was studied over a 40-hour period in a small tidal creek on the south shore of Long Island, New York.2. Measurements were made at frequent intervals of incident radiation, light penetration, salinity, temperature, dissolved oxygen, pH, concentration of diatom cells and their pigments, and dissolved inorganic nutrients. Photosynthesis and respiration were measured by oxygen changes in bottle experiments and estimated from in situ oxygen changes and from chlorophyll α and radiation.3. The plankton community appeared to be nutrient-limited and consisted of a static, non-growing diatom population which was being slowly diluted by tidal action. This was indicated by the diatom counts, the behavior of their pigments (which increased throughout the day and decreased during the night) and the concentration of available plant nutrients.4. Rates of primary production measured by three methods showed good agreement, the values ranging from 3.2 to 5.3 grams carbon assimilated/m2./day on June 26, from 5.1-6.2 on June 27. Total incident radiation for the two days was 300 and 740 gram calories/cm2./day, respectively, and the efficiency of the photosynthetic utilization of visible radiation for the two days was estimated at 2.6% and 1.6%, respectively.