Transplantation and ablation experiments have led to the generalization that in insects the mesoderm is naive, and that pattern is imposed upon it by the ectoderm. This has been demonstrated directly by mosaic analysis for the case of one muscle in Drosophila. The unique character of this muscle depends on the activity of sex-determining and homeotic genes, not in the muscle itself, but in the nerve that innervates it. Indirect evidence suggests, however, that homeotic genes specify some aspects of mesoderm patterning autonomously. Homeotic genes are expressed in the mesoderm, and are regulated in a segment-specific pattern analogous to, but different from, that seen in the ectoderm. Moreover, the effects of homeotic mutations on the muscles do not always mirror transformations seen in the epidermis. Here we examine this problem directly, by expressing homeotic genes ectopically in the mesoderm without altering their expression in the overlying ectoderm. We find that the pattern of adult muscle precursor cells characteristic of the thorax can be converted to that seen in the abdomen by expressing the homeotic gene abdominal-A specifically in the mesoderm.