We have used a monoclonal antibody to examine the distribution of Ultrabithorax (Ubx) proteins in Drosophila embryos and imaginal discs by immunofluorescence. Ubx proteins are nuclear and show a spatially restricted distribution in the nervous system, epidermis and mesoderm. Labelling extends from the first thoracic segment (T1) to the eighth abdominal segment (A8) in the midline cells, from T2 to A8 in the ventral nervous system and epidermis and from A1 to A8 in the somatic mesoderm. In the nervous systems and epidermis the patterns of labelling exhibit a repeat unit, the Ubx metamere, that is out of phase with the segmental repeat unit. At least in the epidermis this repeat unit appears to extend between anterior-posterior compartment boundaries and consists of a posterior compartment together with the succeeding anterior compartment. The most prominently labelled metamere in the nervous system and epidermis is that comprising the posterior region of T3 and the anterior region of A1. Within each metamere the nuclei are heterogeneously labelled. Clear heterogeneity of labelling is also seen amongst the nuclei of the T3 imaginal discs.