Boundary formation and epithelialization are crucial processes in the morphological segmentation of vertebrate somites. By a genetic screening procedure with zebrafish, we identified two genes, integrinalpha5 (itga5) and fibronectin (fn), required for these processes. Fibronectin proteins accumulate at somite boundaries in accordance with epithelialization of the somites. Both Fibronectin accumulation and the epithelialization are dependent on itga5, which is expressed in the most medial part of somites. Although somite boundaries are initially formed, but not maintained, in the anterior trunk of the mutant embryos deficient in either gene, their maintenance is defective at all axial levels of embryos deficient for both of these genes. Therefore, Integrinalpha5-directed assembly of Fibronectin appears critical for epithelialization and boundary maintenance of somites. Furthermore, with an additional deficiency in ephrin-B2a, the segmental defect in itga5 or fn mutant embryos is expanded posteriorly, indicating that both Integrin-Fibronectin and Eph-Ephrin systems function cooperatively in maintaining somite boundaries.