Various lines of evidence suggest that 'segment-polarity' genes, which participate in segmenting the Drosophila embryo, may also play a role in the development of adult structures. Wilkins and Gubb (1991, Dev. Biol. 145, 1-12) have proposed that these genes specify the angular component of the polar coordinate system for each imaginal disc. If true, then segment-polarity mutations should cause abnormal patterning within well-defined sectors of the discs. To test this prediction, a leg segment was used where abnormalities can be precisely identified. The second-leg basitarsus bears eight rows of mechanosensory bristles, plus five chemosensory bristles between specific rows. Abnormalities were sought in the basitarsi of six different segment-polarity mutants and their 15 pairwise compounds. Consistent with the prediction, sectorial defects were indeed found: (1) deletions of specific rows (or chemosensory bristles) or portions thereof and (2) increased bristle number within (or between) specific rows. Both types of abnormalities were associated with changes in the widths of various parts of the circumference, implying that the alterations in bristle patterning may be mediated by regional perturbations of growth.