The first complete amino acid sequence of a neurofilament protein has been established. Porcine NF-L contains 548 residues corresponding to a molecular mass of approximately 62 kDa. This value is noticeably smaller than the 68-72 kDa estimates from gel electrophoresis. Sequence comparison among the 6 non-epithelial intermediate filament (IF) proteins of warm-blooded vertebrates shows that the three NF proteins are the most remote members. Additionally and unexpectedly they reveal among each other lower sequence identity than the three non-neuronal IF proteins GFAP, desmin, and vimentin where the last two are particularly closely related. Certain schemes of IF protein evolution are discussed.