The histones of Caenorhabditis elegans (Nematoda) have been identified by correlating criteria of electrophoresis and amino acid composition with the five main histones from calf thymus. C. elegans H1(1) consists of at least two subtypes with approximate molecular weights of 20,000 and 18,500 daltons as resolved by SDS polyacrylamide gel electrophoresis. They are some 10% smaller than the two subtypes of calf histone H1. The differences are also corrobated by the amino acid composition of the nematode and calf H1 complements. Nematode H2A resembles calf H2A in chromatographic and electrophoretic properties and in the amino acid composition, although it lacks histidine, which seems to be replaced by lysine. Like calf H2A, it is dimorphic as shown by Triton/acid/urea polyacrylamide gel electrophoresis. The H2B complement from C. elegans consists of two proteins with a molecular weight of approximately 12,500. They can be separated by ion-exchange chromatography, but they are very analogous to each other and to calf H2B in amino acid composition. Each form is also resolved into two more subtypes by Triton/acid/urea polyacrylamide gel electrophoresis. Nematode H3 resembles calf thymus H3 in its electrophoretic behaviour; three subfractions can be distinguished in Triton/acid/urea gels. C. elegans H4 is very similar to calf H4 in its chromatographic, electrophoretic and solubility properties, but differs significantly in composition. The meaning of this difference is discussed with regard to the generally observed stringent conservation of H4 sequences between distantly related species.