Photophosphorylation in vivo by Chlorobium limicola was inhibited by lipophilic cations and the energy-transfer inhibitors diphenylphosphorylazide, Dio-9, 4-chloro-7-nitrobenzofurazan and chlorhexidene. Membrane-bound ATPase activity was also inhibited by these energytransfer inhibitors. The formation of a membrane potential was stimulated approximately 1 -7-fold on illumination, rising to a value between -1 10 and -150 mV. The sensitivity of the processes producing this membrane potential to uncouplers, energy-transfer inhibitors and 2-heptyl-4-hydroxyquinoline-N-oxide was measured in the light and the dark.