This study deals with the variations in growth rate and their causation in field populations of the polymorphic landsnail, Cepaea nemoralis (L.) (Pulmonata, fam. Helicidae). The pattern of giowth (measured as increase of shell size) and the influence of several external and of endogenous factors on the growth rate and its variation was, in addition, studied experimentally in the laboratory and the field. The aim was to assess the influence of variation in growth rate as a determinant of the numerical strength of Cepaea populations. Growth rate was found to vary considerably between the individuals in each of the two populations compared. The same holds for adult size, which correlates positively with growth rate. The abiotic factors temperature, humidity, light and calcium were found to influence growth rate in a roughly positive way. Growth rate and adult size were both negatively correlated with population density; the presence of mucus on the substrate contributes to this effect. Growth rate and adult size of the parents and of their progeny are positively correlated; a genetical factor was proved to contribute to this relation. Positive correlations also existed between growth rate (and where studied also adult size) and migration and the reproductive variables number of clutches, clutch size, egg weight and hatching percentage, whereas growth rate and mortality were negatively correlated. It is concluded that the variability of growth rate and changes in it must be of considerable importance in the dynamics of Cepaea populations. Density also directly influences migration, mortality and reproduction.