The reproductive strategies of six species and two subspecies of European vipers are compared. Eighty eight clutches were examined. The reproductive effort, defined as the ratio of total litter weight over body weight of the female just after parturition, varies widely among individuals. This individual variation is in part due to the variable amount of fat accumulated by pregnant females during vitellogenesis, and therefore is associated with their nutrition during the gestation period. However, a genetic polymorphism cannot be ruled out in certain populations. The average reproductive effort of the various species studied is 0.476 ; it does not significantly differ between the species concerned. Among the taxa studied, the average female body weight, just after parturition, ranges from 12.1 to 139.0 g, whereas that of the neonates ranges from 2.74 to 7.4 g, and their number from 2.0 to 8.9 per litter. Therefore the variation in neonatal body weight is smaller than that of the number of newborn per litter. Broadly speaking, small viper species bear fewer offsprings than larger ones, but these offspring are proportionately bigger. The distribution of the reproductive effort not only varies with the body size of the individual adult female ; it also depends on the species concerned, more particularly on the feeding habits and feeding opportunities of the newborn. Those which feed on invertebrates and, to a lesser extent, on small lizards, can afford to be small and numerous. This is not the case for the species whose young feed mostly on small mammals. Within a given taxon, the size of a litter generally increases with the body weight of the mother, whereas there is no correlation between the individual body weight of the newborn and that of their mother. The mortality rate of the young vipers during their first ten months of life varies greatly within the species concerned, at least in captivity. However, within a given species, the larger the newborn, the lower the mortality rate. In conclusion, the individual body weight of the newborn, and the variability of the reproductive effort among the various species of European vipers studied, depend on a number of different factors : female body size, feeding habits of the newborn, availability of prey, and mortality rates of the young in the postnatal period. The values observed under natural conditions obviously result from a compromise between different and often conflicting selective pressures.