The runt gene is required in a developing Drosophila embryo for proper segmentation. Mutant embryos fail to hatch but secrete a larval cuticle in which pattern defects are apparent. In runt embryos, there are pattern deletions spaced at two segment intervals along the antero-posterior axis of the animal. The deleted regions are replaced by mirror-image duplications of the remaining regions. This paper investigates the localized requirements for runt+ activity by analyzing the segmentation patterns in larval genetic mosaics. This analysis is aided by the faintoid and shavenbaby mutations which affect larval cuticle morphology without affecting segmentation. These two mutations serve as markers of the regions of larval cuticle secreted by genetically runt cells. The analysis of the runt mosaic patterns indicates the effects of the gene on segmentation are primarily cell autonomous. This includes both the pattern deletions and the associated mirror-image duplications. This indicates the mirror-image duplications are not due to regeneration but result from a more direct effect of runt on patterning in the embryo. The mosaic patterns also reveal other aspects of the process of pattern formation in the larval epidermis. Based on these results a model is presented for the generation of the larval pattern.