The runt gene is required in a Drosophila embryo for normal segmentation. We investigate this requirement by analyzing runt mutations of varying strength and by manipulating wild-type gene dosage. Elimination of runt causes periodic deletions in the segmentation pattern which are spaced at two segment intervals along the antero-posterior axis. The pattern deletions produced by partial loss of function mutations and by halving the normal wild-type gene dosage reveal a gradation in the requirement for runt, with the centers of the affected regions being most sensitive to deletion. Significantly, increased runt+ dosage causes an anti-runt phenotype consisting of periodic pattern deletions that are out of phase with those caused by runt mutations.