During the second half of embryogenesis, the ellipsoidalCaenorhabditis elegansembryo elongates into a long, thin worm. This elongation requires a highly organized cytoskeleton composed of actin microfilaments, microtubules and intermediate filaments throughout the epidermis of the embryo. This architecture allows the embryonic epidermal cells to undergo a smooth muscle-like actin/myosin-based contraction that is redundantly controlled by LET- 502/Rho kinase and MEL-11/myosin phosphatase in one pathway and FEM-2/PP2c phosphatase and PAK-1/p21-activated kinase in a parallel pathway(s). Although actin microfilaments surround the embryo, the force for contraction is generated mainly in the lateral (seam) epidermal cells whose actin microfilaments appear qualitatively different from those in their dorsal/ventral neighbors. We have identified FHOD-1, a formin family actin nucleator, which acts in the lateral epidermis.fhod-1mutants show microfilament defects in the embryonic lateral epidermal cells and FHOD-1 protein is detected only in those cells.fhod-1genetic interactions withlet-502,mel-11,fem-2andpak-1indicate thatfhod-1preferentially regulates those microfilaments acting withlet-502andmel-11, and in parallel tofem-2andpak-1.Thus, FHOD-1 may contribute to the qualitative differences in microfilaments found in the contractile lateral epidermal cells and their non-contractile dorsal and ventral neighbors. Different microfilament populations may be involved in the different contractile pathways.