The primary structures of the major coat proteins from If1 and Ike filmentous coliphages have been determined by automated Edman degradation before and after cyanogen bromide cleavage and by manual sequencing of certain tryptic peptides. Carboxypeptidase A and B digestion was also used to determine the sequence of the COOH termini of these proteins. A comparison of the major coat proteins from these two phages with those from other filamentous phages show that they all share several common features, namely an asymmetric distribution of positively and negatively charged amino acid residues, which are clustered with the COOH-terminal and NH2-terminal regions respectively, and a region of 19 residues which is located in the middle of the polypeptide chain. The consequences of this charge distribution for a possible mechanism of virus maturation are discussed.