The frequency of the ring-breathing (C,-C, stretching) Raman line of bacteriochlorophyll a (BChl) in the T I state (abbreviated as ur") was determined in 16 different solvents. The u," frequencies were in the region of 1578-1581 cm-l in polar solvents forming the hexa-coordinated monomers, while they were in the region of 1585-1591 cm-1 in polar solvents forming the penta-coordinated monomers. The u," frequencies were 1597 and 1599 cm-i in methylene chloride and carbon tetrachloride forming penta-coordinated BChl aggregates. Possible mechanisms in which the Y," frequency reflects the states of intermolecular interactions are discussed.The Y," frequency of BChl that was bound to the light-harvesting complex (LHC) of Rhodobacter sphaeroides R26 was found to be 1598 cm-I , a result which suggests that a pair of BChl molecules form a dimer in the LHC in the T I state.