This report describes the characterization of the nuclear lamin CeLam-1 of the nematode Caenorhabditis elegans by molecular analysis of the corresponding complete cDNA and gene sequences. The primary structure of CeLam-1, representing only the third non-vertebrate lamin sequence currently known, follows essentially the features displayed by the B-type lamins of vertebrates and Drosophila. The nematode lamin shows, however, some exceptional properties. First, it lacks the SPTR sequence in front of the coil 1a domain which constitutes the major mitotic cdc2 kinase phosphorylation site. Second, two prominent deletions occur in the CeLam-1 sequence. One eliminates 14 amino acid residues from the coil 2 domain. A larger deletion of approximately 25 residues results in the shortest lamin tail domain documented so far. The latter corresponds to a region which varies considerably in sequence from highly acidic in vertebrate B-type lamins to rather basic in Drosophila lamin Dmo. CeLam-1 is encoded by a single 2.3 kb mRNA which is abundantly expressed in mixed-stage worm populations. The 5'-end of the mRNA is generated by trans-splicing to the SL1 leader sequence. The CeLam-1 gene extending over 2.7 kb is located on chromosome I. The gene is composed of 6 exons and 5 short introns, which all interrupt the coding sequence. Surprisingly, none of the intron positions has a counterpart in either the Drosophila lamin Dmo or the vertebrate lamin genes.