The filamentous bacteriophages fd, If1, IKe, Pf1, Xf and Pf3 in aqueous solutions of low, moderate and high ionic strength have been investigated as a function of temperature by laser Raman difference spectroscopy. By analogy with Raman spectra of model compounds and viruses of known structure, the data reveal the following structural features: the predominant secondary structure of the coat protein subunit in each virus is the alpha-helix, but the amount of alpha-helix differs from one virus to another, ranging from an estimated high of 100% in Pf1 to a low of approximately 50% in Xf. The molecular environment and intermolecular interactions of tyrosine, tryptophan and phenylalanine residues differ among the different viruses, as do the conformations of aliphatic amino acid side-chains. The foregoing features of coat protein structure are highly sensitive to changes in Na+ concentration, temperature or both. The backbones of A-DNA and B-DNA structures do not occur in any of the viruses, and unusual DNA structures are indicated for all six viruses. The alpha-helical protein subunits of Pf1, like those of Pf3 and Xf, can undergo reversible transitions to beta-sheet structures while retaining their association with DNA; yet fd, IKe and If1 do not undergo such transitions. Raman intensity changes with ionic strength or temperature suggest that transgauche rotations of aliphatic amino acid side-chains and stacking of aromatic side-chains are important structural variables in each virus.