Expanding urbanization in America has created many new artificial sources which serve as focal points for urban, opportunistic and disease-vectoring mosquitoes. One such urban source, building construction sites, has received little attention despite a growing construction industry. During the summer of 1985, 522 unfinished buildings, 90% as single-family homes, were surveyed for water accumulation, larval mosquito occurrence and density, and species involved. Sump-pits and basements proved to be the most abundant sites of water accumulation of unfinished homes in the later stages of completion with Culex restuans and Cx. pipiens dominating the collections from these sites. Buildings at early stages of construction were much less productive, probably due to sun-exposure. Factors which favor mosquito colonization such as reduced illumination and temperatures, and increased organic content are discussed. Various cultural and insecticidal controls of mosquitoes breeding at construction sites are suggested.