Increasing rates of relative sea—level rise (RSL) have been linked to coastal wetland losses along the Gulf of Mexico and elsewhere. While such losses have yet to be reported for New England tidal marshes, rapidly rising RSL may still be affecting these systems. Studies of the Wequetequock—Pawcatuck tidal marshes over four decades have documented dramatic changes in vegetation that appear to be related primarily to differential rates of marsh accretion and sea—level rise. Other environmental factors such as sediment supply and anthropogenic modifications of the system may be involved as well. When initially studied in 1947—1948 the high marsh supported a Juncus gerardi—Spartina patens belting pattern typical of many New England salt marshes. On the most of the marsh complex the former Juncus belt has been replaced by forbs, primarily Triglochin maritima, while the former S. patens high marsh is now a complex of vegetation types–stunted Spartina alterniflora, Distichlis Spicata, forbs, and relic stands of S. patens. These changes are documented by vegetation sampling that closely followed the 1947—1948 methods and by peat core analysis. Marsh elevations were determined by leveling, and the mean surface elevation of areas where the vegetation has changed is significantly lower than that of areas still supporting the earlier pattern (4.6 vs. 13.9 cm above mean tide level). The differences in surface elevation reflect differences in accretion of marsh peat. Calculations based on sandy overwash layers deposited during historically recorded storms as well as on experimentally placed marker horizons of known age indicate that stable areas have been accreting at the rate of local sea—level rise, 2.0—2.5 mm/yr at least since 1938; changed areas have accreted at about one half that rate. Lower surface elevations result in greater frequency and duration of tidal flooding, and thus in increased peat saturation, salinity, and sulfide concentrations, and in decreased redox potential, as directly measured over the growing season at both changed and stable sites. It is proposed that these edaphic changes have combined to favor establishment of a wetter, more open vegetation type dominated by to distinctive communities–Stunted S. alterniflora and forbs. Changes documented on the Wequetequock—Pawcatuck system have been observed on the other Long Island Sound marshes and may serve as a model for the potential effects of seal—level rise on New England tidal salt marshes.