In Drosophila, the misexpression or altered activity of genes from the bithorax complex results in homeotic transformations. One of these genes, abd-A, normally specifies the identity of the second through fourth abdominal segments (A2 to A4). In the dominant Hyperabdominal mutations (Hab), portions of the third thoracic segment (T3) are transformed toward A2 as the result of ectopic abd-A expression. Sequence analysis and deoxyribonuclease I footprinting demonstrate that the misexpression of abd-A in two independent Hab mutations results from the same single base change in a binding site for the gap gene Krüppel protein. These results establish that the spatial limits of the homeotic genes are directly regulated by gap gene products.