(1) The biology of two populations of Salicornia europaea agg. from the upper and lower levels of Stiffkey salt marsh on the north Norfolk coast was examined. Although seeds of both populations germinate in spring, upper-marsh seedlings grow little until July. In contrast, individuals from the lower marsh show continuous growth throughout the summer. (2) These differences in phenology are maintained when reciprocal transplants are made, or when nitrate or ammonium salts, or sea water, are added to permanent field plots. The delayed growth of individuals is interpreted as a genetic response of the population to the adverse effects of hypersalinity in the upper marsh. (3) The demography of the two populations was examined. Permanent quadrats were mapped or photographed to follow the fate of seedlings; the reproductive and growth performances of individuals in relation to plant density were measured. (4) Overall, demographic trends in the two populations were similar. The probability of a seed giving rise to an adult plant was very low; most seeds died without germinating. Mortality of plants appeared to be density-independent, but there was a significant negative density-dependent relationship between number of seeds per plant and the density of Salicornia plants. (5) By midsummer the seed bank in the sediments was exhausted, so each generation of Salicornia at both sites appears to be distinct. (6) A model is given which describes the population dynamics of Salicornia in terms of density-dependent regulation of seed number and density-independent mortality. The reasons for the prevailing densities are discussed.