The present climate warming exerts significant effects on coastal marine communities. These effects include species ranges moving poleward, declining numbers at the warm edge and increasing numbers at the cold edge of the distribution areas. We describe an example of such changes. We monitored the dynamics of two temperature-sensitive bivalve species for almost half a century in a tidal-flat area in the Wadden Sea . In this area, Abra tenuis is favored by high temperatures andshows high mortality in cold winters. On the other hand, Limecola balthica suffers there from high temperatures and benefits from cold conditions. We tested the hypothesis that the abundance of the former species is increasing, whereas the numbers of the latter one are declining. Our monitoring results corroborated this hypothesis. From literature data, we show that the distribution area of A. tenuis is expanding to the northeast, whereas that of L. balthica is shrinking at the southern edge of its range. We suggest that climatewarming is the common cause of all of these changes. We expect thatA. tenuis will ultimately become more numerous than L. balthica in the Wadden Sea and south of this area.