The sequences of the first 25 residues of histone III, and the first 22 residues of histone IIb2, from trout testis have been determined on an automatic protein sequencer. The amino-terminal sequence of trout-testis histone III is identical to the corresponding region of calfthymus histone III, whereas the trout-testis histone IIb2 sequence differs from that of calf-thymus histone IIb2 at several positions in the amino-terminal region. Several in vivo sites of acetylation of these trout-testis histones have also been determined, by the same automated procedure. In addition to the two main sites at lysyl residues 14 and 23 acetylated in calf-thymus histone III, a lower degree of acetylation at two other sites, lysyl residues 9 and 18, has been detected. Four sites of acetylation have also been detected in trout-testis histone IIb2, at lysyl residues 5, 10, 13, and 18. When the amino-acid sequences around the acetylated lysyl residues of different histones are compared, striking similarities are seen. The methods used in these studies should prove useful in elucidation of the locations of chemically stable modifications in other proteins.