A maternal-effect sex-transformation mutant, transformer (tra), of the housefly is described. It is located on autosome 4 in close linkage with the Ba locus. Normally, the sex of Musca domestica is determined by the presence or absence of an epistatic factor, M. When produced by tra/tra mothers, a large fraction of the tra/tra genotypic female progeny carrying no M factors are transformed to develop into intersexes or fertile phenotypic males. The tra/+ progeny are also transformed, but less frequently. Aging of the mothers increases the frequency of sex-transformed flies. When produced by tra/+ mothers, tra/tra progeny (but not +/tra) occasionally undergo sex transformation. Thus, tra(+) is active both maternally and zygotically. Genotypic males carrying the M factor are not affected by the tra mutant. It is concluded that the tra(+) gene product is required for female determination and/or differentiation. A model is proposed to explain actions of all the known sex-determination genes in M. domestica , and it is discussed in relation to sex-determination mechanisms in several other insect species.