With one exception, phosphoribulokinase from the Rhodospirillaceae requires reduced nicotinamide adenine dinucleotide for maximum activity. This mode of regulation is unique to the facultatively anaerobic photoorganotrophic photosynthetic bacteria, since the phosphoribulokinase from oxygen-evolving photosynthetic species is not subject to activation by reduced nicotinamide adenine dinucleotide. The enzyme was purified of fructose bisphosphatase activity from Rhodopseudomonas capsulata by means of affinity chromatography and was shown to have a native molecular weight of about 220,000. The homogeneous enzyme is composed of a single size polypeptide of 36,000 molecular weight. This study represents the first time the subunit structure of phosphoribulokinase has been determined from any source.