Caenorhabditis elegans has a single lamin gene, designated lmn-1 (previously termed CeLam-1). Antibodies raised against the lmn-1 product (Ce-lamin) detected a 64-kDa nuclear envelope protein. Ce-lamin was detected in the nuclear periphery of all cells except sperm and was found in the nuclear interior in embryonic cells and in a fraction of adult cells. Reductions in the amount of Ce-lamin protein produce embryonic lethality. Although the majority of affected embryos survive to produce several hundred nuclei, defects can be detected as early as the first nuclear divisions. Abnormalities include rapid changes in nuclear morphology during interphase, loss of chromosomes, unequal separation of chromosomes into daughter nuclei, abnormal condensation of chromatin, an increase in DNA content, and abnormal distribution of nuclear pore complexes (NPCs). Under conditions of incomplete RNA interference, a fraction of embryos escaped embryonic arrest and continue to develop through larval life. These animals exhibit additional phenotypes including sterility and defective segregation of chromosomes in germ cells. Our observations show thatlmn-1 is an essential gene in C. elegans, and that the nuclear lamins are involved in chromatin organization, cell cycle progression, chromosome segregation, and correct spacing of NPCs.