CRUSTACEANS and insects share a common origin of segmentation^1,2, but the specialization of trunk segments appears to have arisen independently in insects and various crustacean subgroups^3,4. Such macroevolutionary changes in body architecture may be investigated by comparative studies of conserved genetic markers^5á¤-7. The Hox genes are well suited for this purpose, as they determine positional identity along the body axis in a wide range of animals^8á¤-10. Here we examine the expression of four Hox genes in the branchiopod crustacean Artemia frandscana, and compare this with Hox expression patterns from insects. In Artemia the three ᤘtrunkᤙ genes Antp, Ubx and abdA are expressed in largely overlapping domains in the uniform thoracic region, whereas in insects they specify distinct segment types within the thorax and abdomen. Our comparisons suggest a multistep process for the diversification of these Hox gene functions, involving early differences in tissue specificity and the later acquisition of a role in defining segmental differences within the trunk. We propose that the branchiopod thorax may be homologous to the entire pregenital (thoracic and abdominal) region of the insect trunk.