Climatic and landscape patterns have been associated with both relative mosquito abundance and transmission of mosquito-borne illnesses in many parts of the world, especially warm and tropical climes. To determine if temperature, precipitation, or degree of urbanization were similarly important in the number of potential mosquito vectors for West Nile virus in the moderately temperate climate of western Washington, mosquitoes were collected using CDC carbon-dioxide/light traps set throughout the Seattle region during the summers of 2003 and 2004. The type and abundance of recovered species were compared to ecological correlates. Temperature and mosquito abundance were positively correlated, while precipitation was not strongly correlated with numbers of mosquitoes. Potential WNV mosquito vectors were most abundant in urban and suburban sites, including sites near communal roosts of American crows (Corvus brachyrhynchos). Exurban sites had the greatest vector species diversity, and Culex pipiens was the most abundant species throughout the region.