Photosynthesis by isolated Codium fragile frond-tip chloroplasts appeared to be inversely related to stromal orthophosphate (Pi) and polyphosphate content. Thus, exogenous 10 mM Pi inhibited photosynthesis by chloroplasts with a relatively high internal phosphate status but stimulated it when internal phosphate status was low. Inhibition of 14CO2 fixation by 5 mM Pi was not accompanied by any significant release of photosynthetically-reduced 14C and may therefore be mediated by processes other than, or in addition to, stromal depletion of metabolites as a result of translocator action. In chloroplasts where photosynthesis was stimulated by exogenous Pi, phosphoglyceric acid or glucose-6-phosphate this effect was reversed by the addition of a second test substrate suggesting translocator involvement in this reversal.