We have examined the distribution of Ultrabithorax (Ubx) proteins in embryos mutant for the zygotic gap class of segmentation genes. Members of this class include hunchback (hb), knirps (kni), and Krüppel (Kr). All three mutations disrupt segmentation in specific regions of the embryo. Mutations in kni and Kr produce complex alterations in the Ubx expression pattern. In hb mutants Ubx is ectopically expressed both anterior and posterior to its wild-type boundaries. Thus, the hb gene may play an important role in the specification of the boundaries of Ubx expression. Using the Ubx protein distribution as a marker for metameric organization and using Hoechst dye to monitor cell death, we could follow early events that lead to the final gap-segmentation phenotype in the larval cuticle.