The process of segmentation in Drosophila is controlled by both maternal and zygotic genes. Members of the gap class of segmentation genes play a key role in this process by interpreting maternal information and controlling the expression of pair-rule and homeotic genes. We have analyzed the pattern of expression of a variety of homeotic, pair-rule, and gap genes in tailless and giant gap mutants. tailless acts in two domains, one anterodorsal and one posterior. In its anterior domain tailless exerts a repressive effect on the expression of fushi tarazu, hunchback, and Deformed. In its posterior domain of action, tailless is responsible for the establishment of Abdominal-B expression and demarcating the posterior boundary of the initial domain of expression of Ultrabithorax. giant is an early zygotic regulator of the gap gene hunchback: in giant- embryos, alterations in the anterior domain of hunchback expression are visible by the beginning of cycle 14. giant also regulates the establishment of the expression patterns of Antennapedia and Abdominal-B. In particular, giant is the factor that controls the anterior limit of early Antennapedia expression.