A series of loss of function alleles at the costal-2 locus is described. Embryos mutant for lethal alleles that are derived from a mutant female germ line display polarity defects on the larval segments. A posterior part of the segmental denticle belt is missing and in its place is a mirror-image duplication of the anterior part including the segment boundary. Maternally rescued embryos are lethal but have normal morphology. Hypomorphic alleles escape to adults that display pattern duplications on the wings and halteres. Dominant gain of function alleles at the Costal-1 locus are also described and data are presented that argue that these are neomorphic and act in trans to impair functioning of costal-2. Some wild-type isoalleles of costal-2 are particularly sensitive to interference from Costal-1 mutations and different combinations of these alleles with Costal-1 can lead to embryos in which the primary embryonic field is disrupted (bicaudal phenotype) and adults with pattern duplications on the anterior compartment of most body segments.