The amount of stabilizing selection undergone by a population of animals at different times of its life can be estimated by comparing the variances of different age samples. This paper describes an investigation into the intensities of selection experienced by populations of the common dog-whelk, Nucella lapillus (L.) exposed on different shores to different degrees of wave action. It was found that the variance of more exposed populations was reduced during life by up to 90%. There was a positive correlation between the intensity of stabilizing selection and the amount of wave action experienced by the whelks. Most of this selection took place during the first year or two of the life of the animals.