The five major fractions of histone in Paramphistomum cervi (P. cervi) were isolated and purified according to the method of Phillips and Johns. This is the first of such isolation from parasite tissues. The histone fractions were identified by the acrylamide disc gel electrophoresis and by amino acid analysis. Amino acid compositions of purified histione fractions indicated that the histones were homologous, if not identical, to the H1, H2a, H2b, H3 and H4 histones of calf thymus. The yield of whole histone recovered was 569mg per 100g of weight of the parasite. This value was slightly larger than that of mammalian tissues. The yield of DNA was 206mg per 100g of wet weight of the parasite. Consequently, the ratio of DNA to histone was 0.37. The relative amounts of 5 histone fractions from P. cervi., i.e., H1, H2a, H2b, H3 and H4 were 13.2%, 3.7%a, 26.9%, 51.5% and 4.7%, respectively. This finding of relatively low proportion of histones H2a, and H4 and high proportion of histone H3 in P. cervi. appeared to be due to the species specificity. The electrophoretic behaviors of each histone fractions were almost similar to those of histones from calf thymus. The apparent difference in the ratio of DNA to histone between P. cervi higher organisms was observed, showing the ratio of the parasite: had approximately one third as much the ratio of the animal. The evidence was presented that P. cervi. contained five major histore fractions which were homologous to corresponding 5 fractions of higher eukaryotes, and this study confirmed earlier observations strongly on the conserved primary sequence of histone proteins in nature.