Drosophila, like many other higher organisms, begins development as a tandem array of more or less identical body segments. During later development, these gradually diverge in morphology until the extensive differentiation of body segments seen in the adult is achieved. The genetic control of this divergence is, by now, moderately well understood. The differentiation of the third thoracic and of all abdominal segments appears to be under the control of a cluster of genes known as the bithorax complex (BX-C) (Lewis, 1978). As reviewed below, the genes of this complex appear to be differentially regulated along the body axis so that each body segment is characterized by a unique subset of active BX-C genes. The activities of these genes are thought to directly determine segmental identity. Recently, it has been proposed that more anteriorly located segments, including those in the head as well as the first and second thoracic segments, are under the control of another gene cluster that has been named the Antennapedia complex (ANT-C) by Kaufman et al. (1980). These authors suggest that the ANT-C genes may function in a manner analogous to that of the BX-C genes.