Four populations of Carex lyngbyei from each of three estuaries with contrasting salinity regimes in northwestern North America (deltas of the Nanaimo, Skagit, and Squamish rivers) were grown in a reciprocal transplant experiment to determine the degree to which morphological differences were genetic or environmentally based. Shoot height, biomass per shoot, and aboveground biomass differences were generally not maintained in the transplant environment, suggesting that genetic control on morphological variation was weak. Transplant site salinity regimes had little effect on shoot density or survival. It is suggested that broadly adapted genotypes, capable of this sort of plastic response, may have a competitive advantage in these fluctuating estuarine environments. Key words: Carex lyngbyei, Pacific Northwest, plasticity, reciprocal transplants.