Environmental conditions, primary production, and zooplankton populations were studied from May, 1969 to November, 1970 at one station in Kungsbacka Fjord, Sweden. The fjord, with an arca of 53 km2, is a moderately polluted estuary, with a small tidal range. Data for primary production and environmental parameters were correlated using Spearman's rank correlation coefficient. The annual rate of primary production in 1970 was about 100 gC·m-2. Carbon fixation was about 80 g·m-2 in May–November in1969 and 1970. The average monthly rate was highest in June, 1970, with 25 gC·m-2; about 15 gC·m-2 was recorded in August–October of both years. Carbon fixation by the phytoplankton was estimated to be about 2,800 tons in the whole fjord in 1970. The average fresh-water inflow to the fjord, amounting to about 13 m3·sec-1, added about 380 tons of organic carbon, 45 tons of nitrogen, and 4.5 tons of phosphorus per month. Primary production displayed strong correlation with temperature at different depths (P<0.05 to 0.001), indicating the sediments to be the most important nutrient source. A total of 19 holoplanktonic zooplankton species was identified, copepods being the dominant group. The highest zooplankton biomass, 800 to 900 mg·m-3, was recorded in June of both years. The production of copepods in May–October was about 1 gC·m-2 in both years. The total secondary production of the zooplankton was calculated as only 1.8 gC·m-2 in 1970.