Ecological surveys of mosquito breeding in diked and drained Cape Cod estuaries were conducted in 1982 and 1983. Production of the locally predominant nuisance species, Aedes cantator, was apparently enhanced in stagnant drainage ditches by acidification, caused by diking and drainage activities, which mediated the oxidation and dissolution of naturally-occurring pyrite to sulfate. High sulfate depressed pH and favored immatures of acid-tolerant mosquitoes by limiting fish and probably invertebrate predators. Breeding of Ae. cantator was restricted almost exclusively to acidified drainage ditches and occurred through-out the summer following rain events of at least 2.5 cm. Adults of this species comprised 72% (1982) and 56% (1983) of all nuisance mosquitoes caught in combination light/CO2 traps set in or near the coastal flood plain throughout the summer. Current water quality and related mosquito problems associated with salt marsh diking and drainage strongly suggest that present water management methods are counterproductive for mosquito control and wetland preservation.