Long Island Sound (LIS) is near the northern limit for the range of year-round populations of the blue crab Callinectes sapidus (Rathbun, 1896). This study sampled blue crabs for 7 y with a trawl in LIS and with traps in the estuarine portion of three rivers in eastern Connecticut. Year-to-year abundance of blue crabs was highly variable. A strong correlation was found between the abundance in the summer and the water temperatures in the prior winter, suggesting that these large interannual fluctuations are caused by varying overwinter survival rates. Winter mortality due to low water temperatures may be the controlling factor in determining the northern range limit for resident populations of blue crabs and for their abundance in northern waters, such as LIS. Long-term trawl surveys in LIS show a shift in abundance from cold-temperate to warm-temperate species, which correlates with rising water temperatures. Longer term studies are required to determine whether the blue crab population in LIS is increasing as a result of global warming. Previous studies have not investigated the blue crab life cycle north of New Jersey, including LIS. This study found that all stages of their life cycle occur in LIS. Male and immature female blue crabs were found primarily in the upper, less saline sections of LIS subestuaries. Mature and egg-bearing females occurred primarily in the deeper, higher salinity waters of LIS. These results are consistent with the blue crab life cycle as described in locations south of LIS, such as Chesapeake Bay.