ABSTRACT The identification and analysis of genes controlling segmentation in Drosophila melanogaster has opened the way for understanding similarities and differences in mechanisms of segmentation among the insects. Homologues of Drosophila segmentation genes have been cloned and their expression patterns have been analyzed in a variety of insects, revealing that the patterns of expression of many genes are conserved. Conserved expression patterns do not, however, necessarily reflect conserved gene function. To address gene function, we have conducted a screen for mutations that alter embryonic patterning of the beetle, Tribolium castaneum. One of the mutations isolated, godzilla, affects early steps in the segmentation process in the whole animal, like Drosophila pair-rule mutants. Another mutation, jaws, is novel: it causes both a dramatic homeotic transformation in the thorax and first abdominal segment as well as a deletion of most of the segments of the abdomen. In Tribolium and other intermediate germ band insects, the anterior segments of the embryo are determined in the syncytium of the blastoderm, whereas the abdominal segments proliferate in a cellular environment. Both the godzilla and jaws mutations affect segments that are formed in the syncytium differently from those that are formed after cellularization. These regionally specific phenotypes may reflect the different patterning mechanisms that must be employed by the anterior and posterior regions of an intermediate germ insect.