: Fluctuating asymmetry has been proposed as a general and sensitive indicator of developmental instability. Although there have been many field studies of fluctuating asymmetry in populations exposed to toxic chemicals, there have been few laboratory studies. To test the hypothesis that stress from toxic chemicals causes an increase in fluctuating asymmetry, we exposed larval Drosophila melanogaster to six concentrations of lead and benzene in their food. Lead and benzene caused neither a significant reduction in the number of emerging adult flies, nor a significant difference in the average number of sternopleural bristles. Flies exposed to lead at 10mg kg(-1) and benzene at 1000 mg kg(-1), however, showed increased fluctuating asymmetry of sternopleural bristles. Higher concentrations (10,000 mg kg(-1)) of benzene caused a transition from fluctuating asymmetry to directional asymmetry. Flies exposed to benzene at 10,000 mg kg(-1) also eclosed more than a day earlier than flies exposed to it at 0-1,000 mg kg(-1).