The seasonal changes of eelgrass biomass and net production were recorded in an eelgrass bed in the Øresund, Denmark, from August 1978 through December 1979. The eelgrass biomass varied from 170 g dry wt • m^-2 in early spring to 622 g dry wt • m^-2 in autumn. The ratio of maximum biomass to minimum biomass was 23 for leaves and 2.5 for roots and rhizomes. Leaf growth was measured in situ by a leaf-marking technique. The leaf growth of individual turions followed a unimodal seasonal pattern, with the highest values in June. During winter, leaf growth was slow but continued even under ice-cover. Variation in light explained 75% of the seasonal variation in leaf growth rates, temperature only 6%, while salinity did not contribute significantly. Maximum leaf area index was constant at the locality in two consecutive years and was independent of turion densities. The annual net production of eelgrass was 2209 g dry wt • m^-2 or 814 g C •- m^-2 which corresponded to 92% of the total eelgrass plus epiphyte production. During autumn there was a considerable loss of eelgrass leaves and turions. This loss is often exported from the ecosystem.