The multitude of sex determination mechanisms displayed in dipteran insects has usually been described in terms of variations on a single principle in which the primary signal of the primitive pathway consists of a single allelic difference at one locus. Evolution of sex determination mechanisms is thought to have occurred by the addition of genes below the top gene of the pathway. The elucidation of the complex sex determination pathway of Drosophila melanogaster, as well as recent evidence that the basal genes of the pathway seem to be conserved across metazoan genera both in structure and, to a lesser degree, in function, points towards the possibility that sex determination pathways may have evolved from the bottom-up. Further to this is the question of whether the dominant male-determining factor, M, which is found in a number of insect species, represents part of the ancient sex determination pathway or is a later addition to the pathway. This, together with the possibility that the M factors found in numerous dipteran insect species may have a common origin, is discussed. The similarities of the sex determination pathways under the control of M and the implications in relation to the construction of genetic sexing strains for biological control are also discussed.