In this paper we characterize the order of hypodermal cell fusions in the Caenorhabditis elegans hermaphrodite. Somatic cell fusions are part of the developmental program of many tissues in a variety of organisms. The formation and remodeling of tissues and organs can be studied at the cellular level in C. elegans. Here we establish a system for studying cell fusion by characterizing somatic cell fusions during morphogenesis in C. elegans. Fusion is a common cell fate in this nematode; numerous epithelial fusions occur in the hypodermis, vulva, uterus, and excretory gland cells (Sulston et al., 1983. Dev. Biol. 100, 64-119). Some but not all pharyngeal muscles also fuse (Albertson and Thomson, 1976. Philos. Trans. R. Soc. London Ser. B 275, 299-325). We have studied the behavior of epithelial adherens junctions before and during cell-to-cell fusions in embryonic and postembryonic development. Our results define the timing and sequence of short-range migrations followed by fusions that generate syncytia. We have made use of an antibody that stains adherens junctions to study the behavior of hypodermal cells during development. Fusion of specific cells in the hypodermis causes rearrangements of the adherens junctions between cells. Fusion events usually start in the anterior part of embryos or larvae. There is some variation in the specific order in which cells fuse, but the final positions, boundaries, and sizes of syncytia are the same. In some cases fusion causes isolation of a mononucleate cell or group of cells by a surrounding, growing syncytium. Our characterization of the order of cell fusions will provide a basis for the identification of molecular events required for regulated membrane fusion during development.