Laboratory experiments were carried out on thread-bearing spat of the tellinid bivalve Macoma balthica, and in the field high numbers of these juveniles (of up to 1 cm shell length) were caught in winter in plankton nets suspended in tidal streams of the Wadden Sea (The Netherlands). These data are used to explain the distribution and dynamics of a population of the species in the North Sea off the West-Frisian island Terschelling. Evidence is presented that nearly all M. balthica arrive in this area in winter by secondary settlement after tidal current transport, facilitated by long hyaline threads. M. balthica were numerous only close to a tidal inlet connecting the area with the Wadden Sea. Further away (more than 10 to 15 km), young M. balthica arrived in low numbers only and were smaller. All or nearly all M. balthica in the North Sea will have originated from massive primary settlement of just-metamorphosed larvae more than half a year previously (in spring and summer) on high tidal flats in the Wadden Sea. Some functional aspects of dispersal by secondary settlement are discussed

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