d-Ribulose-1,5-bisphosphate carboxylase/oxygenase has been purified 80-fold from malate-grown Thiocapsa roseopersicina by salting out the enzyme from the high-speed supernatant between 68–95% saturation with respect to (NH4)2SO4, gelfiltration through Sephadex G-100, and DEAE-cellulose chromatography followed by sedimentation into a 14–34% glycerol gradient. The specific activity of enzyme for the carboxylase reaction was 2.45 μmol RuBP-dependent CO2 fixed/min · mg protein (at pH 8.0 and 30° C) and for the oxygenase reaction was 0.23 μmol RuBP-dependent O2 consumed/min · mg protein (at pH 8.6, and 25° C). The enzyme, which was ultracentrifugally homogeneous in the presence of 4 and 10% v/v glycerol, was stable for at least one year at-80° C in the presence of 10% glycerol. S20, w values obtained in the presence of 4 and 10% glycerol were 19.3 and 16.2, respectively. The enzyme contained both large (53,000-daltons) and mixed small subunits (15,000- and 13,500-daltons).