armadillo (arm) is one of a group of Drosophila segment polarity genes that are required for normal patterning within the embryonic segment. Although arm RNA is uniformly distributed in embryos, arm protein accumulates at higher levels in regions that contain wingless, another segment polarity gene which encodes a secreted protein that regulates patterning via cell-cell communication. These local increases in arm protein require wingless activity, and mutations that alter wingless distribution produce corresponding changes in the arm protein pattern. These results suggest that wingless regulates accumulation of arm protein by a posttranscriptional mechanism. Two other segment polarity genes, porcupine and dishevelled, are required for this effect. We also show that arm protein is closely associated with the plasma membrane in virtually all cell types and often colocalizes with F-actin.