This study analyzes changes in water quality of the Naugatuck River as a result of an upgrade to the Waterbury Wastewater Treatment Plant (WWTP). Data gathered by Connecticut's Department of Environmental Protection (DEP), as well as by the author during summer 2000 and 2001, were used and statistical analyses were conducted on this data. The Biological Oxygen Demand (BOD) levels, ammonia-nitrogen levels, and survival rates of test organisms for the effluent from the WWTP before, during, and after construction were analyzed. Ammonia, BOD, and dissolved oxygen (DO) levels in the river, downstream from the outfall, were analyzed for differences before, during, and after construction, with distance from the outfall, and with drainage area. The macroinvertebrate community that was sampled downstream from the WWTP was analyzed for the levels of sensitive taxa before, during and after construction. Ammonia and BOD levels did not make any significant improvements in the river after compared to before construction but DO was improved. However, all of these water quality indicators met Connecticut water-quality standards in the river, but before construction they did not meet all standards. The macroinvertebrate community after construction in terms of sensitive taxa remains consistent with levels before construction. The DEP should continue monitoring of the river to determine if further improvements to biotic communities in the river are made as a result of the upgrade.