The fourth chromosome mutant cubitus interruptus‐Wallace(ciW) produces leg, wing, and body bristle aberrations. The effect on the wing is similar to that produced by cubitus interruptus‐dominant (ciD) which also has an influence on larval segmentation indicating that it has a regulatory function. Leg morphology of haplo‐4, ciW, and mosaic haplo‐4:diplo‐4, ci/ci+ flies was examined in an attempt to distinguish between a structural and a regulatory function by ciW. Aberrations recovered include failure of segment elongation, intersegmental gaps, duplication of bristles, and segments that are shorter than normal and of greater than normal diameter. Many of these effects are localized, suggesting that ciW may act to maintain cell positional reference. Increased local cell proliferation appears to be one manifestation of loss of the normal function.