odd Oz (odz), the only pair-rule gene that does not encode a transcription factor, is expressed in every imaginal disc and imaginal tissue of Drosophila melanogaster, as assessed by immunocytochemical staining for the protein and by odz-enhancer trap line staining. The eye imaginal disc expresses odz at the morphogenetic furrow, in R7 photoreceptor cells, and in adepithelial cells. Genetic evidence indicates that odz is involved in furrow progression, and in R7 cell RTK receptor tyrosine kinase signalling. Expression in the wing pouch appears in a complex pattern that displays a symmetry axis at the dorsal-ventral boundary. In other dorsal thoracic disc sites, Odz appears at specific points of prospective thoracic integument and prospective proximal structures of the wing and haltere. Expression in the leg and antennal discs is strongest in prospective proximal segments at sites homologous between these discs. In addition, odz appears in sensory mother cells (SMCs) in all of the above-mentioned discs. This recurrence indicates the general importance of odz in the development of the peripheral nervous system (PNS), which complements the finding that it is expressed in the CNS, with striking optic lobe staining. odz must be involved in cellular signalling in pleiotropic roles in varied pathway contexts.