In the Drosophila embryo the establishment and specification of metameric units depends upon the selective activation of the segmentation and the homoeotic selector genes. The former are necessary for establishing the appropriate number of metameric or parasegmental units, whereas the latter control the pathways of differentiation followed by particular parasegments. Classical embryological manipulations have show n that these processes must be closely coordinated during normal development. However, previous studies of pair-rule genes have led to the suggestion that the specification of segmental identity proceeds independently of the establishment of metameres as physical units. These apparently conflicting perspectives can be reconciled by envisaging a common maternally derived positional information system which is independently interpreted by the components of both processes. In the case of the partitioning process, the gap and pair-rule genes would be instrumental in translating this information, whereas the activation of the homeotic genes would be mediated via other intermediaries (see ref. 9 for review). It is difficult to see, however, how such a system could ensure the precise regulation of the tw o types of genes implicit in the final differentiated pattern. This difficulty has led to the suggestion that the segmentation mechanism must define the precise boundaries of selector gene expression. Here we confirm this suggestion and propose that the gene fushi tarazu plays a key role in this process, integrating the processes of metameric partitioning and regional specification in the Drosophila embryo.