A spill of fuel oil at West Falmouth, Massachusetts, in 1969 contaminated contiguous salt marshes with up to 6000 μg oil/g (ppm) of wet mud and affected local populations of the salt-marsh crab Uca pugnax. Directly related to high sediment oil content were reduced crab density, reduced ratio of females to males, reduced juvenile settlement, heavy overwinter mortality, incorporation of oil into body tissues, behavioral disorders such as locomotor impairment, and abnormal burrow construction. Concentrations of weathered fuel oil > 1000 ppm were directly toxic to adults, while those of 100–200 ppm were toxic to juveniles. Cumulative effects occurred at lower concentrations. Recovery of the marsh from this relatively small oil spill is still incomplete after 7 yr. Interpretation of the crab population data upon the basis of detailed analysis of the aromatic fraction of the fuel oil provided by Dr J. Teal, Woods Hole Oceanographic Institution, showed that recovery of the crab populations was highly correlated with the disappearance of the naphthalene fraction of the aromatics. There had been a decrease in the aromatics from 42% in 1970 to approximately 18% in 1976 in Station I surface sediments, while preliminary analyses show aromatics still high at other stations where little recovery has been observed. By 1970–71 all parent naphthalene compounds were gone in Station I surface sediments, but substituted naphthalenes were still in high concentrations. By 1972–73 dimethyl naphthalenes and C3 and C4 substituted naphthalenes remained at about 25% of original values. By 1976–77 only C3 and C4 substituted naphthalenes remained at less than one tenth of their 1973 levels. In 1972–73 the substituted naphthalenes were at high enough concentrations to prevent recruitment, as large juvenile crab settlements in 1970–73 produced no recruitment into the crab populations. By 1976–77 these toxic compounds were at low enough levels that recovery of the crab population was occurring with recruitment, and increasing density was observed in both years. High aromatic concentrations at other stations may still be inhibiting recovery at these stations. The long-term inhibition of recruitment and low population densities may have resulted from exposure to oil in the interstitial waters during the sensitive molt period and/or during the long periods of time while the crabs were overwintering in the substrate. Key words: petroleum, Uca pugnax, crab, salt-marsh, population, pollution