We identified a seven-pass transmembrane receptor of the cadherin superfamily, designated Flamingo (Fmi), localized at cell-cell boundaries in the Drosophila wing. In the absence of Fmi, planar polarity was distorted. Before morphological polarization of wing cells along the proximal-distal (P-D) axis, Fmi was redistributed predominantly to proximal and distal cell edges. This biased localization of Fmi appears to be driven by an imbalance of the activity of Frizzled (Fz) across the proximal/distal cell boundary. These results, together with phenotypes caused by ectopic expression of fz and fmi, suggest that cells acquire the P-D polarity by way of the Fz-dependent boundary localization of Fmi.