IN species where males and females differ in number of sex chromosomes, the expression of sex-linked genes is equalized by a process known as dosage compensation. In Drosophila melanogaster, dosage compensation is mediated by the binding of the products of the male-specific lethal (msl) genes to the single male X chromosome. Here we report that the sex- and chromosome-specific binding of three of the msl proteins (MSLs) occurs in other drosophilid species, spanning four genera. Moreover, we show that MSL binding correlates with the evolution of the sex chromosomes : in species that have acquired a second X chromosome arm because of an X-autosome translocation, we observe binding of the MSLs to the ’new’ (previously autosomal) arm of the X chromosome, only when its homologue has degenerated. Moreover, in Drosophila miranda, a Y-autosome translocation has produced a new X chromosome (called neo-X), only some regions of which are dosage compensated. In this neo-X chromosome, the pattern of MSL binding correlates with the known pattern of dosage compensation.