A new approach was made to comparing the contributions to response of chromosomes 2 and 3 of Drosophila in lines selected for high and low sternopleural bristle number.Separate response curves for chromosomes 2 and 3 were obtained from changes in the effect of standard second and third chromosomes marked Cy and Mé which were kept segregating with their wild-type homologues during selection.Dominance interaction between marker and wild chromosomes caused some bias in estimating responses in each direction, but the amount by which the responses diverged in the two directions of selection was relatively free from bias. On a log. scale divergences of chromosomes 2 and 3 were in the ratio 1∶1.7 and their heritabilities realised early in selection were 0.14 and 0.26 respectively.There was interaction between the Cy and the Mé chromosomes which was not altered by the selection. Almost all the responses in the lines could be accounted for by addition of the responses in single chromosomes 2 and 3, chromosomes 1 and 4 making a negligible contribution.Sampling, as a cause of variation between selection lines, was reflected in the variation between them in response in chromosomes 2 and 3.