The effect of the extra Y chromosome on the number and fluctuating asymmetry of sternopleural bristles in Drosophila melanogaster was studied at two temperatures, 18 degrees C and 25 degrees C, in the stock In(l)wm4/y+Y. The extra Y chromosome significantly increased the number of sternopleural bristles especially in the lateral areas of the sternopleura, which hampered the analysis of the effect of the extra Y chromosome on fluctuating asymmetry. The formula used was one which assumes that the fluctuating asymmetry is dependent on the total number of bristles. It was found that the extra Y chromosome increased fluctuating asymmetry at 18 degrees C but decreased it at 25 degrees C. There was less fluctuating asymmetry in females than in males. It appears that the extra Y chromosome has an effect on the temperature which is most favourable to the development of the sternopleural bristles.