The suitability of polytene chromosomes for population genetic and in situ DNA physical mapping was investigated in Johannesburg, South Africa, Culex quinquefasciatus Say and Culex pipiens L.. The most defined and consistent polytene spreads were obtained from salivary glands of early fourth instars from Cx. quinquefasciatus. A photomap is provided for a Cx. quinquefasciatus colony originating from Johannesburg. Variations in banding patterns, chromosome aberrations, and paracentric inversions within the colony and comparisons with previous published descriptions are discussed. Chromosomes of sufficient quality were obtained such that a DNA coding for an odorant binding protein could be mapped by colorimetric in situ hybridization.