Culex restuans Theobold, a putative vector of West Nile virus among birds in northern North America, also may serve as a bridge vector to mammals. Despite its potential public health importance, little is known about what regulates populations of this species. Mosquitoes generally are subject to both density-dependent reductions in survival and growth and to density-independent limitations on their population abundances. The mechanisms by which density dependence may occur in this species were examined in both field and laboratory studies. Nutrient-dependent reductions in growth were found in field studies. Under laboratory conditions, nutrient levels in larval habitats and total water volume per container contributed to survival and growth of larvae. We related these findings to density-independent changes in available habitat for larval development observed in other studies. These results may suggest a mechanism for patterns of mosquito abundance.