As part of our effort to induce and identify mutations in all genes on chromosome 4 of Drosophila melanogaster, we have mapped the breakpoints of eight chromosome 4 deficiencies relative to the predicted genes along this chromosome. Although the approximate locations of Df(4)G, Df(4)C3, Df(4)M101-62f, Df(4)M101-63a, Df(4)J2, Df(4)O2, Df(4)C1-10AT, and Df(4)B2-2D are known (some from cytological observations and others predicted from P element locations), the extents of these deletions have not been mapped with respect to the predicted genes identified by the Drosophila Genome Project. Polymerase chain reaction primers were designed to amplify the predicted exons of all chromosome 4 genes, and homozygous embryos for each deficiency were identified and their DNA used to test for the presence or absence of these exons. By testing for the inability to amplify various exons along the length of the chromosome, we were able to determine which predicted genes are missing in each deficiency. The five deficiencies, Df(4)G, Df(4)C3, Df(4)C1-10AT, and Df(4)B2-20 (all terminal deletions), and Df(4)M101-62f (a proximal interstitial deletion), enabled us to partition the gene-containing, right arm of chromosome 4 into five regions. Region A [uncovered by Df(4)M101-62f] contains the proximal-most 21 genes; region B [uncovered by Df(4)B2-2D] contains the next 12 genes; region C [uncovered by Df(4)B2-2D and Df(4)C1-10AT] contains the next 17 genes; region D [uncovered by Df(4)B2-2D, Df(4)C1-10AT, and Df(4)C3] contains the next 21 genes; and region E [uncovered by Df(4)B2-2D, Df(4)C1-10AT, Df(4)C3, and Df(4)G] contains the distal-most ten genes. By using Df(4)M101-62f, Df(4)B2-2D, Df(4)C1-10AT, Df(4)C3, and Df(4)G in complementation tests, we can assign newly induced recessive lethal mutations to one of the five regions on chromosome 4. This will substantially reduce the amount of DHPLC analysis required to match each mutation to a predicted transcript on chromosome 4.