The incorporation of (32)P and (14)C into organic compounds by Ankistrodesmus is strongly inhibited by X-rays. In the same phosphorylated compounds (32)P-incorporation apparently is more severely inhibited by X-rays than the (14)C-labelling. The (32)P-incorporation into organic compounds is more strongly inhibited than (32)P-labelling of inorganic phosphate in the cell. The inhibition of (32)P-incorporation into a number of compounds is strikingly uniform. It is concluded that the inhibition of (32)P-incorporation and of (14)C-incorporation into phosphorylated compounds in vivo is due to an uncoupling by X-rays of photophosphorylation as in vitro. The difference in X-ray sensitivity of (14)C- and (32)P-incorporation into one organic phosphorous compound is attributed to a dual action of X-rays on (32)P-incorporation in organic compounds (both via the uncoupling of photophosphorylation) and only a single effect on (14)C-incorporation and (32)P-labelling of inorganic phosphate. The effect of X-rays on (14)C-incorporation into organic compounds included inhibition in most cases but also stimulation as in the case of glycolic acid. These differences may be due to interference in the intercellular regulations following the application of X-rays. The inhibition of (14)C-incorporation in many cases exhibits different behaviour at low (<200 krad) and high doses. These changes are discussed on the assumption that at the lower doses X-rays cause uncoupling of photophosphorylation and at the higher doses an additional inhibition of electron transport.