In Drosophila the process of segmentation depends on the function of coordinate, gap, pair-rule and segment-polarity genes. Mutations in segment-polarity genes cause defects in the pattern of every segment. Here the cloning of sequences from a segment-polarity gene, wingless, and the in situ localization of a transcript in embryos are described. The transcript is first detected in the anterior and posterior regions of the blastoderm embryo at cellularization, and accumulates in a series of stripes in the extended germ band, one stripe per metameric unit. Each stripe is localized to the most posterior cells of each parasegment. The signal is predominantly epidermal, and transcript accumulates only transiently in the mesoderm and nervous system. This pattern of expression is discussed with respect to models of pattern formation in segmental units.