Viviparidae display two categories of life history traits – one constant and independent of habitat, another depending on ecological conditions. The relatively constant abundance is ensured by clustering in the same places at the same time, sex ratio (prevalence of females), size structure (presence of all size classes, with the largest snails forming the majority), and the high proportion of gravid females in the population. These traits undergo only seasonal variation in particular habitats. Adjustment to environmental changes involves the number of embryos per female and the size of reproducing females. Long-term life history studies were performed on Viviparus viviparus (L.) and V. contectus (Mill.) from various habitats (dam reservoir, river, oxbows connected to the river to varied extent) in Poland. The habitats differed in their surface area, depth and trophic status. The snails from the dam reservoir, ecotone zones in the river and stagnant oxbow lakes reproduced at a large body size, their fecundity being size-dependent. In very variable habitats, like oxbow lakes periodically joined to the river, viviparids reproduced at a younger age and smaller body size, while their fecundity did not increase with the body size. The reproduction was the most important factor determining the viviparid density in the studied habitats. This was mainly associated with the ovoviviparity which allowed for controlling the reproduction process. Reproduction is a flexible life history trait in the populations of V. viviparus and V. contectus.