The Brey model to calculate secondary production from data on biomass and mean individual weight is widely used, but so far it has not been tested for communities in which one or a few species dominate. On the base of a long-term (tens of years) data series on estimates of densities, biomass, mean individual weights and annual production of the most important bivalve species (together accounting for about 60% of the zoobenthic biomass) in a vast tidal-flat area, we compared production values obtained by two methods: the weight-increment summation method and the Brey model. In 4 out of the 5 studied bivalve species, the Brey estimates were on average substantially lower than the direct estimates. As a consequence, the Brey method seriously underestimated the production of the entire bivalve community in about half of the years. In 10 out of the 28 years, the Brey estimates were even less than half of the actual bivalve production. The one species (Limecola balthica) with a satisfactory match between the two methods was (as contrasted to the 4 other species: Cerastoderma edule, Mytilus edulis , Mya arenaria , Ensis leei ) characterized by low annual weight increments and the simultaneous presence of many year classes, none of them strongly dominating. As secondary production of the studied ecosystem is dominated by the group of large bivalves, Brey estimates for such bivalve-dominated systems are inadequate for a study of their functioning.