The diurnal variation in the activity of ribulose-l,5-bisphosphate carboxylase (RuBPCase), the major CO2-fixing enzyme in plants, has been shown to result from the influx and efflux of Mg2+ ions into and out of the chloroplast stroma. A recent re-examination of the phenomenon indicates that the inactivation of the enzyme, rather than being due to the efflux of Mg2+, is correlated in some plant species with an increase in the concentration of an organic phosphate ester in the chloroplast in the dark1–3. We have purified this potent inhibitor from dark-treated potato (Solanum tuberosum) leaves, and established that its structure is 2-carboxy-D-arabinitol-1-phosphate, a molecule that closely resembles an intermediate in the carboxylase reaction of RuBPCase.