Limb development in Drosophila requires the activity of a proximo-distal pattern-forming system, in addition to the antero-posterior and dorso-ventral pattern-forming systems that subdivide the embryo. Several lines of genetic evidence indicate that the Distal-less gene plays an important part in specifying proximo-distal positional information. The Distal-less locus encodes a homoeodomain-containing protein, which suggests that Distal-less may exert its activity through differential regulation of subordinate genes. The spatially restricted pattern of Distal-less expression allows direct visualization of the limb primordia during early embryogenesis. Here I report that from their inception, the leg primordia span the parasegment boundary. The segment polarity gene wingless seems to have a key part in defining the positions at which leg primordia will develop along the antero-posterior axis of the embryo. This analysis allows a direct molecular visualization of the compartments that subdivide the limb primordia into discrete developmental domains.