The yellow (y) mutation of Drosophila melanogaster affects the development of behavior and morphology. We have analyzed some behavioral and morphological parameters during the development of y mutants. Wild-type third instar larvae move in straighter paths than larvae of the same age homozygous for the y mutation. At 96 h of age, the tracks of y larvae have 10 times as many loops as tracks of wild-type larvae, and at 120 h of age, y larvae show bending behavior about 2.5 times more frequently than do wild-type. Consequently, they do not disperse as much as wild-type larvae. Concomitant with the behavioral changes, the larvae present a defect in the morphology of large chaetae in the larval denticle belts, particularly of 2nd and 3rd instars, both with light and scanning electron microscopes. These results suggest that a cuticular defect is probably involved in the abnormal locomotor activity observed in y larvae of Drosophila melanogaster.