Culex pipiens pipiens is comprised of two forms: Cx. pipiens pipiens form pipiens and Culex pipiens pipiens form molestus. One population of Cx. p. pipiens form molestus in the United States, located in the 91^st Street sewer of New York City (NYC), has been relatively well-characterized. The objective of this review is to summarize the results of multiple independent studies on Cx. p. pipiens form molestus in NYC and present the current understanding of the origin of this population. While solid evidence exists supporting the genetic differentiation and population structuring between Cx. p. pipiens form pipiens and form molestus in NYC, the possibility that the 91^st St. sewer population of form molestus was founded by local aboveground Cx. p. pipiens form pipiens cannot be ruled out based on available data to date. Future studies in the US should include sympatric populations of both forms of Cx. p. pipiens from around the world in order to provide the necessary context to resolve these phylogenetic relationships.