The cDNA coding for calf filensin, a membrane-associated protein of the lens fiber cells, has been cloned and sequenced. The predicted 755-amino acid-long open reading frame shows primary and secondary structure similarity to intermediate filament (IF) proteins. Filensin can be divided into an NH2-terminal domain (head) of 38 amino acids, a middle domain (rod) of 279 amino acids, and a COOH-terminal domain (tail) of 438 amino acids. The head domain contains a di-arginine/aromatic amino acid motif which is also found in the head domains of various intermediate filament proteins and includes a potential protein kinase A phosphorylation site. By multiple alignment to all known IF protein sequences, the filensin rod, which is the shortest among IF proteins, can be subdivided into three subdomains (coils 1a, 1b, and 2). A 29 amino acid truncation in the coil 2 region accounts for the smaller size of this domain. The filensin tail contains 6 1/2 tandem repeats which match analogous motifs of mammalian neurofilament M and H proteins. We suggest that filensin is a novel IF protein which does not conform to any of the previously described classes. Purified filensin fails to form regular filaments in vitro (Merdes, A., M. Brunkener, H. Horstmann, and S. D. Georgatos. 1991. J. Cell Biol. 115:397-410), probably due to the missing segment in the coil 2 region. Participation of filensin in a filamentous network in vivo may be facilitated by an assembly partner.