Cell movements and shape changes of the hypodermal cells during embryogenesis in C. elegans appear to drive morphogenesis of the body. At about 200 min of development, the hypodermal cells initially located on the dorsal surface of the embryo extend ventrally and anteriorly. Thus the embryo becomes enclosed in a monolayer of hypodermal cells that are linked to each other by adherens junctions. Immediately following hypodermal enclosure, the hypodermal cells begin to shorten along the circumferential direction and lengthen in the longitudinal (anteroposterior) dimension as the entire embryo elongates approximately four-fold. Circumferential actin filament bundles form in all five rows of hypodermal cells (one dorsal, two lateral, and two ventral) at the start of elongation and appear to contract to generate the force that transforms the shapes of the hypodermal cells and the embryo. We have isolated several zygotic lethal mutations in a gene named hmp-1 (humpback) that seem to specifically block elongation of the embryo. hmp-1 mutants form the normal pattern of hypodermal cells but arrest elongation at an early stage (1.25- to 1.5-fold) and develop abnormal lumps in the dorsal hypodermis. Analysis of a genetic deficiency indicates that this phenotype results from complete loss of zygotic gene function. Curiously, the primary defect in hmp-1 embryos appears to be failure of the dorsal hypodermis to elongate. The ventral (and possibly also lateral) hypodermis elongates initially, presumably forcing the dorsal hypodermis into ectopic folds. Circumferential actin filament bundles form properly in the hypodermal cells of hmp-1 embryos. However, at the start of elongation, the filaments in the dorsal hypodermis pull away from the adherens junctions where they normally remain anchored. We positioned the hmp-1 mutations with respect to the physical map and obtained transgenic rescue with cosmid DNA. Analysis of cDNA clones in this region from the genome sequencing project revealed that one cDNA hybridized to the rescuing cosmid. Injection of anti-sense RNA made from this cDNA into the gonads of wild-type animals produces the Hmp-1 phenotype in embryos, indicating that this cDNA encodes hmp-1. Extending Y. Kohara's initial sequence of this clone, we find that it appears to be a partial cDNA encoding the C-terminal one-third of an a-catenin homologue (52% identical to Drosophila a-catenin). a-catenin is a component of vertebrate adherens junctions that binds to the cytoplasmic side of cadherins (transmembrane, homotypic cell adhesion proteins) and to F-actin. Our mutant analysis suggests that, in some cells, a-catenin is required to anchor actin filaments at adherens junctions and transmit force from the cytoskeleton to the membrane, and ultimately to neighboring cells. hmp-1 RNA is expressed zygotically in all hypodermal cells before hypodermal enclosure of the embryo. Zygotic expression is also observed in the developing intestine and pharynx, tissues containing adherens junctions. In addition, hmp-1 RNA is expressed maternally and present in all early blastomeres. We have begun to examine phenotypes of embryos lacking both maternal and zygotic hmp-1 gene activity using germline mutant clones and transgenic strains. In these embryos, the hypodermis fails to fully enclose the anterior and ventral regions, possibly due to defects in cell adhesion or migration. The intestine also shows defects that might possibly result from defective cell polarity. We thank Andreas Wissmann for providing one of the hmp-1 alleles.