The erythrocyte histones of rainbow trout were compared with those of goose by polyacrylamide gel electrophoresis. A band analogous to goose erytbrocyte-specific histone V, but not identical in relative mobility or quantity, was found to be a component of trout erythrocyte histone. A similar component was also found in carp erythrocyte histone, but it was absent from trout liver histone. To reveal this band clearly, it was advantageous to displace the histone III monomer by oxidation.To verify the character of this protein, each of the main erythrocyte histones of trout were purified by chromatography on Amberlite CG-50, eluted with guanidinium chloride, and then further purified by exclusion chromatography on Bio-Gel P-60. Amino acid compositions of corresponding trout and goose histones, including that of the erythrocyte-specific histone, were sufficiently similar to establish their analogous identities. In general, the chromatographic and electrophoretic properties of histones I, IIb1, IIb2, and V from trout differed more from those of goose, than did their gross amino acid compositions. Comprehensive fractionation and characterization is necessary to establish identities of corresponding histone fractions.An extensive quantitative variability was found among erythrocyte-specific histones of fish. This must be reconciled with hypothetical roles for this histone in erythropoiesis.