Embryos homozygous for Krüppel die as late embryos with an altered segmentation pattern. In strong alleles the normal thoracic and anterior abdominal segments are replaced by a partial mirror image duplication of the posterior abdomen. Weak alleles cause smaller pattern deletions in the thorax and abdomen and are not associated with mirror image duplications. The altered segmentation pattern can be traced back to 12 min after the onset of gastrulation, when the shorter germ bands in homozygous Kr embryos provide a first indication of abnormal patterning. The mutant was mapped to position 107.6 at the tip of the right arm of the second chromosome, cytologically to bands 60F25. Analysis of homozygous deficiency embryos indicate that the phenotype produced by strong point mutations probably represents the amorphic condition. The requirement for Kr⁺ gene activity is strictly zygotic. Maternal dosage of Kr⁺ has no effect on the embryonic phenotype, nor does homozygosity for Kr prevent germ cells from making normal eggs capable of normal embryonic development when fertilized by wild-type sperm. The requirement for Kr⁺ seems restricted to embryogenesis. Homozygous clones induced in imaginal discs during larval development survive and develop normally and in vivo cultures established from homozygous embryos proliferate normally and metamorphose into adult structures of normal morphology.