Prolific growth of Cyanophyceae in regions apparently lacking ordinary plant food prompted the investigation of sp. of Nostoc, Rivularia, and Gloeocapsa. Artificial media best suited to their growth were investigated and the following cultures were started: Rivularia and Nostoc in Detmer- Moore solution on sand; Nostoc in Knop solution on sand; and Gloeocapsa in artificial sea water on sand. After some time, tests for N showed an appreciable increase in the 4 series. Bacteria were observed in the algal mucus. Incubated series showed considerable fixation of atmospheric N by the bacteria collectively; Several well-known N-fixing organisms were identified in large numbers[long dash]Azotobacter chroococcum, Clostridium pastorianum, Bacillus radicicola. It was concluded that N-fixing bacteria inhabit the mucus of some Cyanophy-ceae to the mutual benefit of both organisms concerned.