Moulting and copulation by the barnacle Tetraclita japonica attached to an intertidal concrete pier were observed continuously with a video camera housed in a plastic case in summer 1996 and 1997. In a total of 779 individual-day observations, 10 individuals became sexually receptive as functional females and 14 moulted. Copulation occurred not only immediately after, but also before moulting. Duration of receptivity varied from 2 min to nearly 12 h, during which functional females accepted between two to 582 penis insertions by between one to 11 surrounding males. Each insertion time was short, ranging from 0.6 to 42 s, with a median of 2.4 s. Sex role changes were observed between two animals located close to each other, the shortest interval for which was 3.3 h. No significant size differences were detected between functional males and females. A temporal pattern of copulation, with a high frequency of penis insertions of short durations, is discussed in relation to natural habitat conditions.