Mean densities, size-class frequency, egg numbers and length/weight relationships were used to estimate production of Daphnia hyalina var. lacustris at weekly intervals from October 1975 to September 1976 in Farmoor I reservoir, mean depth 4.6 m to 8.9 m. Densities ranged from 3/1 to 116/1, biomass from 0.06 mg dry wt/1 (0.22 g C/m2) to 1.67 mg dry wt/1 (4.59 g C/m2). Greatest reproductive activity occurred in spring 1976 when a maximum of 32% of the population was gravid resulting in a maximum of 27 eggs/litre. Net rate of population growth was variable, but was high and positive in early spring and negative as summer progressed. Three methods were used to calculate annual production. Estimates were from 15.4 g C/m2/year to 68.6 g C/m2/year. The lowest value was derived from biomass and population turnover times and was subject to underestimation of the birth rate. The highest value was derived from the rate of individual life-cycle stage weight increases and the abundance of each stage. This method suffers from an inadequate knowledge of the relationships between the development time and environmental factors. An intermediate estimate of production, 57.6 g C/m2/year was obtained by assessing the grazing and assimilation rates for a known standing crop using laboratory derived filtering rates.