A method has been developed for the isolation of nuclei from the flagellated protozoan Cuglena gracilis, and histone Isolated from the nuclei has been extensively studied. The amino acid composition of the unfractionated Euglena histone is similar to that for the histone of higher organisms. Euglena histone was separated by gel filtration into five fractions and the individual fractions were further purified by selective precipitation and solubility techniques. Amino acid analysis of the purified proteins indicated the presence of f1, f2b, f2al and f3 histones. The remaining fraction, designated f2a2, differed from vertebrate f2a2 in having a higher lysinet arginino ratio. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate indicated that the sizes of the individual Euglena histones are similar to the homologous vertebrate fractions. Using electrophoretic techniques, a comparison was made of Euglena histone and histones isolated from rat liver, trypanosomatid organisms and the colourless euglenold Astasia longa. The nature and complexity of the histones from the various organisms were broadly similar, but some differences in the electrophoretic patterns were evident.