Conserved semaphorin-plexin signaling systems govern various aspects of animal development, including axonal guidance in vertebrates and epidermal morphogenesis in Caenorhabditis elegans. Here we provide in vivo evidence that stimulation of mRNA translation via eukaryotic initiation factor 2␣ (eIF2␣) is an essential downstream event of semaphorin signaling in C. elegans. In semaphorin/plexin mutants, a marked elevation in the phosphorylation of eIF2␣ is observed, which causes translation repression and is causally related to the morphological epidermal phenotype in the mutants. Conversely, removal of constraints on translation by genetically reducing the eIF2␣ phosphorylation largely bypasses requirement for the semaphorin signal in epidermal morphogenesis. We also identify an actin-depolymerizing factor/cofilin, whose expression in the mutants is predominantly repressed, as a major translational target of semaphorin signaling. Thus, our results reveal a physiological significance for translation of mRNAs for cytoskeletal regulators, linking environmental cues to cytoskeletal rearrangement during cellular morphogenesis in vivo.[Keywords: C. elegans; cofilin; eIF2; epidermal morphogenesis; mRNA translation; semaphorin] Supplemental material is available at http://www.genesdev.org.