Aphanocapsa 6308 metabolizes both NaHCO3 and Na2CO3. The short term incorporation (5-s) metabolic pattern and the patterns of incorporation of bicarbonate for exponential versus stationary phase cultures differ, however. Cells were equilibrated for 10 min in air and distilled water prior to injection of either NaH14CO3 at pH 8.0, or Na214CO3 at pH 11.0. Hot ethanol extracts were analyzed via paper chromatography and autoradiography for products of CO2 fixation. At 5 s, malate (51.5%) predominates slightly as a primary bicarbonate fixation product over 3-phosphoglycerate (40.3%); 3-phosphoglycerate is the primary product of carbonate fixation. At 60 s, the carbonate and bicarbonate labelling patterns are similar. Cells in stationary phase fix in 5 s a greater proportion of bicarbonate into malate (36% vs. 14% for 3-phosphoglycerate) than do cells in exponential growth. Likewise, 60 s incorporations show a large amount of bicarbonate fixed into aspartate (30.9%) in stationary phase cells over that of exponential phase (11.6%). These data suggest an operative C4 pathway for purposes not related to carbohydrate synthesis but rather as compensation for the incomplete tricarboxylic acid cycle in cyanobacteria. The enhancement of both aspartate fixation and CO2 fixation into citrulline in stationary phase correlates with an increase in cyanophycin granule production which requires both aspartate and arginine.