Adenylate cyclase activity was examined in a variety of organisms using a highly sensitive assay. Activity was found in a blue-green alga, four green algae, two cellular slime molds, a fungus and moss protonemata. Fern prothalli and fronds gave variable results. No activity was detected in any of the higher plant tissues tested. The results throw further doubt on the existence of adenosine 3':5'-cyclic monophosphate in higher plants.