Richter1 and Fewson et al.2 have both reported that fructose diphosphate (FDP) aldolase activity could not be demonstrated in blue-green algae, especially Anacystis nidulans. A peculiar steady-state growth condition induced by uric acid as sole nitrogen source and characterized as a severe nitrogen deficiency has been found in a marine species of blue-green algae: Agmenellum quadriplaticum, strain ‘PR-6’. In this condition 45 per cent of the cell dry weight was a glucose polymer3. Certain enzyme systems in the uric acid grown cells of ‘PR-6’ were examined in an attempt to gain some idea of the possible pathway for glucose formation. In the course of these experiments an active aldolase was found. This communication deals with the demonstration and some characteristics of the aldolase found in crude extracts of Anacystis nidulans, with supporting data from ‘PR-6’ and one other blue-green alga.