Abstract The McIntosh apple in storage was found to produce malic acid by fixation of carbon dioxide at a rate of about half of the net daily acid loss. The contribution of this reaction to the production of amino‐acids and protein was also quantitatively significant. The fixation activity rose to a high level at the time of harvest and continued to be high throughout the storage period. The rate of uptake of carbon dioxide increased with rising concentration of carbon dioxide in the external atmosphere.