ABSTRACT SUMMARY The mesoderm of Drosophila embryos is segmented; for instance there are segmentally arranged clusters of cells (some of which are heart precursors) that express even-skipped. Expression of even-skipped depends on Wingless, a secreted molecule. In principle, Wingless could act directly in the mesoderm or it could induce the pattern after crossing from ectoderm to mesoderm. Using mosaic embryos, we show that Wingless produced in the mesoderm is sufficient for even-skipped expression. This proves that induction is not essential. However, induction can occur: when patches of wingless mutant mesoderm are overlaid by wild-type ectoderm, they do express even-skipped. We therefore believe that Wingless from both the ectoderm and mesoderm may contribute to patterning the mesoderm. Using the UAS/Gal4 system, we made embryos in which the Wingless protein is uniformly expressed. This is sufficient to rescue the repeated clusters of even-skipped expressing cells, although they are enlarged. We conclude that the mesoderm is segmented in some way not dependent on the distribution of Wingless, suggesting a more permissive and less instructive role for the protein in this instance.