This report contains a description of the karyotypes of Musca sorbens Wied., Musca vetustissima Walker, several strains of the subspecies domestica and the subspecies curviforceps and calleva of Musca domestica L., Musca autumnalis De G., Muscina.stabulans Fall., Orthellia nudissima (Loew), Ophyra leucostoma Wied., Phaonia basalis Fabr., and Phaonia variegata Fabr. Species M. stabulans, O. nudissima, and P. variegata have 2n = 10 chromosomes and all the others 2n = 12, but some specimens of M. domestica curviforceps have only 11. The sex chromosomes of M. autumnalis and O. leucostoma are very small and this pair has apparently been lost in the 10-chromosome species. It is suggested that species having 10-chromosome karyotypes may have originated independently in different subfamilies and genera of the Muscidae.