Photochemical activity was examined in membrane fragments and a purified membrane preparation from Chloroflexus. Flash-induced absorption difference spectroscopy strongly suggests a primary donor (P865) that is more similar to the P870 bacteriochlorophyll a dimer found in the purple photosynthetic bacteria than it is to P840 found in the anaerobic green bacteria. Redox measurements on P865 and an early acceptor also indicate a photochemical system characteristic of the purple bacteria. The membrane preparation contains a tightly bound type c cytochrome, c554, that is closely coupled to the reaction center as indicated by its ability to rereduce photooxidized P865. Chloroflexus thus appears to be distinct photochemically from other families of photosynthetic bacteria and may occupy an important role in photosynthetic evolution.