On the tidal flats in the Wadden Sea mussels Mytilus edulis form extensive epibenthic beds. Near the island of Sylt (German Bight, North Sea), these extend from mid intertidal to shallow subtidal zones and are often heavily overgrown by barnacles (Semibalanus balanoides and Balanus crenatus). The effects of tidal emergence and barnacle epibionts on growth in M. edulis were investigated by field experiments and surveys. The results showed that mussel cohort lengths increased from the mid intertidal (25-35 mm shell length) over low intertidal (30-45 mm) to the subtidal zone (45-60 mm). In the low intertidal zone M. edulis were heavily overgrown by barnacles (2.2 ± 0.4 g dry weight mussel-1), while balanid epigrowth was significantly lower subtidally (0.9 ± 0.4 g mussel-1) as well as in the mid intertidal zone (0.7 ± 0.3 g mussel-1). Cross-transplantation experiments of clean mussels (25-35 mm length) showed that from July to September growth was about 3 mm in the intertidal zone and significantly increased to about 10 mm subtidally. Mussel origin had no influence upon growth. Further field experiments showed significantly lower growth in mussels with barnacles than without. Living and experimentally sealed barnacles did not differ in their effect, indicating that food competition was not responsible. This investigation suggests that fluctuations in the extent of barnacle epigrowth may modify growth patterns in M. edulis which are otherwise governed by tidal exposure. © 2001 Elsevier Science B.V.