The aerobic nitrogen-fixing unicellular cyanobacterium Synechococcus RF-1 (pcc 8801) contains numerous irregular polyglucan granules ranging in size from 0.1 to 0.4 mu m. These morphologically unusual piagetian granules are not found in the sheathed nitrogen-fixing unicellular cyanobacterium Gloeothece RF-6 (pcc 8803). Two forms of alpha-amylase, ethyleneglycolbis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)-sensitive and EGTA-insensitive, were found in the Synechococcus RF-1. In addition to their EGTA sensitivity, the two forms of alpha-amylase also differed in their reactive pH range and in their zymogram disc gel electrophoresis. Each form of alpha-amylase was stable and constant in concentration through a diurnal light/dark cycle.