The function of the frizzled (fz) locus in Drosophilia melanogaster is required to coordinate the cytoskeletons of epidermal cells to produce a parallel array of cuticular hairs and bristles (for example on the wild-type wing all hairs point towards the distal tip). In fz mutants it is not the structure of individual hairs and bristles that is altered, but their orientation with respect to their neighbours and the organism as a whole. Mitotic clone analysis indicates that fz has two functions in the developing wing. It is required for the proximal-distal transmission of an intercellular polarity signal, a process that is expected to be at least partly extracellular. It is also required for cells to respond to the polarity signal, which is expected to be a cytoplasmic function. The fz locus could encode either one bifunctional or two single-function proteins. We report here that, in pupae, fz produces a messenger RNA that encodes a protein with seven putative transmembrane domains. Thus, the Fz protein should contain both extracellular and cytoplasmic domains, which could function in the transmission and interpretation of polarity information, respectively. This is the first reported sequence for the protein product of a tissue polarity gene.