Activation of the Drosophila EGF-receptor (DER) is spatially and temporally controlled by the release of its various ligands. DER and its ligand Spitz mediate the formation of specific somatic muscle precursors. We show that a second DER ligand, Vein, complements the activity of Spitz in the development of various somatic muscle precursors. In vn mutant embryos, the DER-dependent muscle precursors do not form in some of the segments. This phenotype is significantly enhanced in embryos carrying only one copy of wild type spitz. Our analysis suggests that Vein activation of DER differs qualitatively from that of Spitz in that it does not lead to the expression of the inhibitory protein Argos, possibly leading to a continuous activation of the DER signaling pathway.