This study examines the population behaviour of the ovo-viviparous mollusc Potamopyrgus jenkinsi in a number of freshwater habitats. Two generations were normally completed annually but a single annual cycle also occurred. Two main overwintering strategies were recorded involving either high mortality of all but small individuals and their subsequent attainment of maturity in late June, or high winter survival with the consequent ability of adult snails to begin significant young production in early May. Reasons for observed variations in the lifecycle are discussed. Large scale migrations from deep to shallow water were recorded in the spring. Reproductive rates in field and experimental populations were investigated on the basis of cage experiments, embryo number and ratio of developmental stages. Reproduction was continuous and was closely correlated with temperature. Growth and mortality rates were also investigated. Snail populations about to undergo heavy winter mortality were characterised by the presence of fully developed embryos only; adult mortality at other times was not accompanied by changes in reproductive condition. Evidence of density effects in the field are discussed. Onset of maturation was characterised by changes in shell configuration in which the aperture formed a small lip or shoulder, precluding further significant growth. Final (maturation) size varied seasonally due to different growth rates, and was greatest in May and J1me and least in August and September. The implications for reproductive rate and development are discussed. Maturation size was combined with a quantitative assessment of shell deposit to determine relative age and survival of adult snails. These indicated that regulation of breeding populations occurred both through sub-adult and adult mortality, the relative importance of these mechanisms varying at different times of year and between generations Records of the patchy and apparently aberrant distribution of P. jenkinsi may be partly attributable to its variable population behaviour but specific habitat requirements, particularly in relation to substratum, were also indicated. The sites examined were to some extent atypical, reaching extremely high densities and resembling estuarine populations of hydrobiids in this respect. A comparison with production levels attained by other freshwater, and marine, molluscs is carried out and related to the probable position of P. jenkinsi in freshwater habitats.