The populations of benthic invertebrates were studied in Alderfen and Upton Broads [England] from Nov. 1971-June 1975 inclusive. Production estimates were made for the last 2 yr. Both lakes are small and shallow. Alderfen Broad was culturally enriched, with a phytoplankton bloom and absence of aquatic macrophytes. Upton Broad was naturally eutrophic, with clear water and a dense growth of Najas marina. Twenty-two spp. or groups occurred in Alderfen Broad, 7 of them regularly. Forty spp. or groups were present in Upton Broad, 17 of them regularly. The greater diversity at Upton Broad was related to the presence of Najas. Potamothrix hammoniensis was abundant in Alderfen Broad (maximum population 22,500 m-2). Chironomus plumosus was also common. At Upton Broad C. tentans occurred and Tanytarsus holochlorus formed seasonally large populations (up to 130,000 m-2). Molluscs, Caenis horaria and Mistacides longicornis were also common at Upton Broad. Annual respiration and production estimates are presented for individual species. The overall production amounted to 99 KJ m-2 and 176 KJ m-2 in the 2 yr at Alderfen Broad. Corresponding values for Upton Broad were 130 KJ m-2 and 273 KJ m-2. Overall P:B ratios were estimated as 2.1 and 1.8 at Alderfen Broad and 2.2 and 3.0 at Upton Broad. Production was higher at both sites in the 2nd year. Biomass and community metabolism remained higher over the year at Alderfen Broad. At Upton Broad biomass and community metabolism was low for most of the year, but was very high from May to July. The significance of these patterns of production to the population of long-lived predators, especially fish, is discussed. A nutrient perturbation in the diverse but essentially fragile ecosystem of a shallow lake such as Upton Broad could result in accelerating eutrophication through positive feedback interactions in the community once environmental heterogeneity is reduced.