This paper reports studies carried out on natural populations of two species of viviparous clams, Sphaerium striatinum and S. simile, at three localities in two mesotrophic lakes in upstate New York. Sphaeriids are dominant members of the benthic fauna at the second trophic level in these localities, and more generally are cosmopolitan and ubiquitous animals.A growth survey involved monthly collections of regular samples, and these were used in measurements of growth, fecundity, mortality, and biomass production. S. striatinum proved to have an 18 to 24 month lifespan and S. simile one of about 26 months. Differences in reproduction included a maximum brood size of 12 in S. striatinum and of only 6 in S. simile, spat recruitment extending over 8 months in S. striatinum and over 9 months in S. simile, and a mean spat size at birth of 3.6 mm in S. striatinum and 6.4 mm in S. simile. The gross fecundity is thus markedly lower than that in the majority of molluscs. Mortality is to some extent compensating, adult survivorship being greater in S. simile, but mortality rates are remarkably steady and relatively low in both species. Field assessments of density showed that S. striatinum was dominant in one locality, and S. simile in another locality in the same lake, but that S. striatinum occurred at greater densities than any other sphaeriid species.Cage experiments confirmed the steady mortality rates and low fecundity values for these sphaeriids. Individual growth rates in cages are considerably higher than the mean values from population samples, since growth rates there are not obscured by recruitment of spat.A more complete analysis of growth in both species involved a series of over 400 determinations of total nitrogen (by microDumas). These were utilized in computations of productivity. Embryonic production makes up only a small fraction (< o per cent) of the biomass turnover in these viviparous sphaeriids. The overall annual productivity (in terms of total nitrogen) is o.8 Dr. W. Junk b.v. Publishers -The Hague, The Netherlands grams nitrogen per square meter per year for S. striatinum and only o.28 grams nitrogen per square meter per year for S. simile.In a discussion of these figures they are shown to correspond to about 33 per cent of the second trophic level productivity for S. striatinum and about o per cent for S. simile. Since sphaeriids are long-lived, these percentages reflect a real dominance. In further discussion of the 'coexistence of these two species', it is pointed out that mortality is higher for the less common species in each locality.