The Drosophila segment polarity gene known as shaggy or zeste white 3 encodes a ubiquitously expressed serine-threonine protein kinase which is critical for a number of important developmental processes. In the developing wing blade, clones of cells lacking the normal shaggy-zeste white 3 product form dense tufts of margin-like bristles and bristle precursors. In a previous study I hypothesized that this phenotype could be best explained as a transformation in the regional identity of wing blade cells to one resembling that found along the normal wing margin. A number of genes have recently been identified which are expressed exclusively or at higher levels along the normal wing margin; in this study I will show that two of these genes, vestigial and scalloped, are overexpressed at margin-like levels in shaggy-zeste white 3 clones. This phenotype does not depend upon the formation of ectopic bristle precursors and occurs in clones lacking both shaggy-zeste white 3 and the entire achaete-scute complex. As vestigial and scalloped are both involved in early patterning events prior to the stages of bristle specification, these results strongly suggest that shaggy-zeste white 3 is required for the normal specification or maintenance of regional identity in the developing wing blade. The margin-like transformation is, however, partial, since the expression of apterous (in pupal wings) and wingless and cut (at late third instar) was not reliably altered in shaggy-zeste white 3 clones. It has been suggested that shaggy-zeste white 3 is involved in a wingless signaling pathway in the embryo; a model is discussed in which shaggy-zeste white 3 acts downstream of localized apterous and wingless expression to specify or maintain margin identity in the wing.