The wingless protein is secreted, but it is not known whether it acts only on cells near to its site of synthesis or whether it has a longer range. Here, we use mosaic Drosophila embryos to estimate the range of wingless as it acts to maintain expression of the engrailed gene. We find that expression of engrailed is often sustained in those wingless- cells that are located near a wildtype patch of tissue, but this is not invariably so. Also, the numbers of cells maintaining engrailed expression are small. From these findings, we argue that wingless-expressing cells sustain engrailed expression only in adjoining cells. The wingless gene is also needed for maintenance of its own expression; using mosaics, we find that the range of this action is short as well.