Observations on the growth of cockles have emphasized the difficulty of choosing a single minimum size to cover a large area of coastline. A study of the mechanism of mesh selection showed that shell width is an important agent of selection by both square-meshed and oblong-meshed sieves, but the effects of selection need to be interpreted in the length of dimension by which growth is measured. Growth rates within a single year-class have been examined, and the effects of time of settlement and rate of growth on the sizes achieved in subsequent years considered. The effects of selection by sieving on the mean lengths of annual growth checks on the shells, from which growth rates may be determined by back-measurement, have been examined. It was concluded that on average slower-growing cockles remain after selection, but the proportion of narrow-shelled apparently faster-growing cockles will increase.