The amino acid sequence of small core protein of bacteriophage phiX174 has been determined by a combination of automated Edman degradation of the intact polypeptide and by analysis of tryptic and thermolytic peptides. The six lysyl and six arginyl residues of this 37-residue polypeptide are concentrated in two structurally homologous 12-residue segments of the sequence. The hydrophobic residues of valine, tryptophan, tyrosine, and phenylalanine are contained in the carboxyl-terminal nine residues of the protein, together with one of the two leucyl residues and two of the three glutaminyl residues. The single free carboxyl group in the protein is the alpha-COOH of the C-terminal phenylalanyl residue. The overall sequence of this small core protein suggests that it may function as a DNA-condensing protein. The protein sequence presented here corresponds exactly to the DNA base sequence of the cistron J region of the phiX174 genome determined in another laboratory.