The chrysophycean alga, Ochromonas ma_hm,sis Pringsheim, was shown to synthesize cyclic adenosine 3':5T-monophosphate (cAMP) and to release it into the culture medium. Cells contained 3 to 3,000 picomoles per gram fresh weight; medium contained up to 20 times the amount in the cels. Putative I32PIcAMP was purified from cultures supplied I32Plphos-phate. The compound was identified as 132PIcAMP by co-chromatography with authentic cAMP through 10 serial steps; by chemical deamination at the same rate as authentic cAMP, to a 32p compound with the chromatographic behavior of cIMP; and by its conversion through the action of cyclic nucleotide phosphodiesterase to a 32p compound with the chromatographic behavior of 5'-AMP. A two-step procedure involving chromatography on alumina and on Dowex 50 purified the unlabeled compound from cells or medium sufficiently for it to be assayable by competitive inhibition of binding of I3HIcAMP to cAMP-binding protein (Gilman assay) or by stimulation of cAMP-dependent protein kinase. The activity was destroyed by cyclic nucleotide phosphodiesterase with the same kinetics as authentic cAMP, provided that an endogenous inhibitor of the phosphodiesterase was first removed by an additional purification step.In bacteria and animals, cAMP4 is often involved in the response of the organism to a change in the supply of a nutrient, particularly the carbon source (18). For example, the level of glucose is elevated in mammals by cAMP-mediated activation of phosphorylase, which generates glucose via phosphorolysis of glycogen. In Escherichia coli and related bacteria, a decline in glucose supply leads to elevation of cAMP, which in turn promotes the synthesis of enzymes that catalyze the utilization of other carbon sources. Because photoautotrophic members of the plant kingdom normally synthesize glucose from CO2, rather than absorb glucose from the environment, the question arises: do these photoautotrophs have need of such a cAMP system? Despite numerous studies on the possible presence and functions of cAMP in photosynthetic organisms, the extent and circumstances of occurrence of this nucleotide in such organisms in the plant kingdom remain uncertain and controversial (1,12). A large part of the confusion is attributable to incomplete character-'