In order to investigate the localized requirements for the activity of genes that are required in a Drosophila embryo for segmentation, we have analyzed the patterns in genetic mosaics. In this paper, we describe our results with four different X-chromosome linked segmentation loci: armadillo, fused, giant and unpaired. For each locus, we first describe in detail the cuticle phenotype of mutant embryos. We then describe the segmentation patterns in embryos mosaic for these mutations, in each case utilizing the shavenbaby (svb) larval cuticle marker mutation to identify the regions of pattern made by genetically mutant cells. For all four loci, we can identify embryos containing large regions of both mutant and wildtype pattern. In these mosaics the regions of mutant pattern are marked with svb and the genetically wildtype (svb (+)) cells make wildtype pattern. The interpretations of the patterns in embryos mosaic for fused and unpaired are complicated by the variability of the phenotypes. However, after taking these complications into account, our principal conclusion is that the requirement for embryonic gene activity seems to be primarily cell autonomous. Based on the descriptions of the mutant phenotypes of these four loci and the analysis of the mosaics, we speculate on the possible roles these genes play in the process of segmentation.