Scanning tunneling microscopy (STM) has been employed to study the organization of photosynthetic bacteria. A high-resolution spatial image is acquired on a bacteriochlorophyllide c (BChlide c) monolayer prepared by a simple method. BChlide c molecules are found to self-organize a well-defined adlayer on a highly oriented pyrolytic graphite (HOPG) surface. A dimeric structure different from those observed in Chls a, and c and BChlide d is first revealed in the adlayer. A hydrogen bond is assumed to exist in the dimer between the OH groups at C-3(1) positions.