Some 300 kb of DNA from the 9F12-10A7 X chromosome region (seven bands) uncovered by Df(1)vL3 were cloned and 31 break points of chromosome rearrangements within the region were mapped. Positions of 12 genes found earlier in genetic saturation experiments, transcripts and P element-induced mutations were located on the physical map using either chromosome rearrangements or Southern blot hybridizations. Data on the position of the break points, genes and polytene chromosome bands allow the following conclusions to be made. (1) The size of the bands in the region varies between 4 kb (10A6 and 7) and 183-195 kb (10A1-2). The compaction ratio of DNA in bands varies from 8-36 (10A6 + 7) to 151-161 (10A1-2). Therefore, fine and thick bands appear to have different kinds of DNP packaging. (2) The bands differ in genetic content. Fine bands contain from one to three genes. In contrast, the 10A1-2 band contains three genes and at least six transcribed DNA fragments. (3) Comparison of genetic and physical maps shows that in this region 0.01 centiMorgan corresponds to 3.3 kb of DNA.