Laboratory based experiments, carried out on Littorina neglecta collected from the barnacle zone at Ravenscar, north-east England, showed that this marine prosobranch displays size assortative mating. Males prefer to mate with larger, more fecund females, but they do not necessarily choose the largest available. It is suggested that such size assortative mating may act as a barrier to reproduction between L. neglecta and L. saxatilis. No relationship was found between either male or female shell size and the length of copulation.