The life history of Trochulus hispidus (L.) over two generations was studied under laboratory conditions. In this semelparous, short-lived species, in which the juvenile lifespan constitutes almost three quarters of the whole life, substantial juvenile mortality is accompanied by a very high reproductive rate. The snails become reproductively mature at the age of one to two years; the growth rate varies greatly even among hatchlings from the same brood; the size of such siblings may differ by as much as two whorls at a single moment of time. These differences result in a spread of the reproductive period within a population over the whole active season. We hypothesize that this may spread risks where mortality is high and may be concentrated in short periods of adverse conditions. While full-sib matings among the offspring generation result in a 60% reduction in the proportion of ovipositing pairs relative to the parental generation, in those that do lay there are no differences in batch size or juvenile survival. It seems possible that avoidance of any inbreeding depression depends on pre-zygotic selection.