The reproductive activity of a population of barnacle-dwelling littorinids at Peak Steel, on the north-east coast of England, was investigated. Variation in the number of mature males and females, expressed as a percentage of the whole population, showed a marked difference among the barnacle-dwelling littorinids. Littorina neglecta displayed a distinct seasonal component to its reproductive activity, with a peak in early summer. Littorina arcana B also displayed a distinct seasonal component, with a lull in the summer months. However, L. saxatilis B maintained its reproductive activity all year, but with an increase in output in the summer. The reproductive activity of both L. arcana B and L. saxatilis B follow similar patterns of reproductive activity as their typical non barnacle-dwelling forms, suggesting that these are true barnacle-dwelling ecotypes. The controversy surrounding the specific status of L. neglecta is also discussed.