Somites are segmentally repeated mesodermal structures within the vertebrate embryo that give rise to the vertebrae and muscle of the trunk and tail. A recent large-scale screen for developmental mutants of the zebrafish, Danio rerio, identified mutations in five complementation groups that affect the anterior-posterior pattern of the somites: fused somites (fss), beamter (bea), after eight (aei), deadly seven (des) and white tail (wit). Both embryological and molecular genetic techniques are being used to further characterize the role that fss, bea, aei, and des play in somitogenesis and to identify the affected genes. Moreover, cross-species experiments will be used to elucidate the extent of any evolutionary conservation of the regulatory networks involved in segmentation. Thus, this analysis is aimed at both understanding the mechanism by which the paraxial mesoderm is segmented in the zebrafish and determining the extent of conservation between species of genetic regulatory networks involved in segmentation.