The aristae of Drosophila have been shown to play a role in mating behavior and geotaxis. Two populations of D. melanogaster were selected for increased and decreased numbers of major aristal branches. Selection was successful and resulted in two lines differing by an average of six aristal branches. Hybridization analyses of selected lines revealed that genes influencing aristal branching are located on both the X chromosome and the autosomes. Polygenic control of aristal morphology is indicated by a gradual response to selection and low realized heritabilities. When selection was relaxed for 19 generations, the number of aristal branches did not revert to the number in the control line. Changes in aristal branching did not appear to have a consistent influence on geotaxis, although there was a tendency for flies with fewer aristal branches to be geonegative. Neither mating speed nor ethological isolation between the two populations was affected by selection. It is concluded that the number of aristal branches in Drosophila is a neutral trait (i.e., not subject to natural selection) under laboratory conditions. Correlations between aristal morphology and behavior found in other selection experiments by previous investigators were likely due to linkage disequilibria.