The literature relevant to mosquito genetics is first reviewed. In the present work, 51 different strains of Aedes aegypti have been examined for colour variation in particular. Methods of rearing, handling and routine are briefly described. Colour of the dorsal abdomen was so variable, that a scheme was devised to classify it, according to paleness, in 37 grades and sub- rades. These could be assiened to 15 numerical colour values to enable the paleness of a population to be quantitatively defined. Photography of variants was adopted as a routine. In Linkage Group I, two partially sex-linked factors controlling eye colour and one controlling abdominal colour were isolated, and 4 linkage distanoes determined. This is the first example of partial sex-linkage in A. aegypti and the first three-point, linkage estimation in any mosquito. In Linkage Group II one new mutant control ing thoracic colour was isolated and its recombination with the previously described s locus measured. At least 4 mutant alleles are shown to occur at the s locus sld one is identical with a gene previously ascribed to another locus. Two new mutants were isolated in Lirucage Group III and the crossover distance of one measured from the previously described blt locus at which a second mutant allele was isolated. A further three potentially useful mutants were obtained and other variation mentioned. The variation in abdominal colour in 39 strains is described and discussed. The frequency of genotypes in populations polyworphic for an s allele suggests,though not significantly, some degree of heterosis. Hybrids were successfully obtained between three pairs of Stego yia species. The relationship between A. aegypti and A. mascarensis, one of the two crosses giving fertile hybrids, is discussed more fully. A number of gynandromorphs and intersexes is also recorded.