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Nitrogen (N) available to plants mostly originates from N 2 fixation carried out by prokaryotes. Certain cyanobacterial species contribute to this energetically expensive process related to carbon (C) metabolism. Several filamentous strains differentiate heterocysts, specialized N 2 -fixing cells. To understand how C and N metabolism are regulated in photodiazotrophically grown organisms, we investigated the role of sucrose (Suc) biosynthesis in N 2 fixation in Anabaena sp. PCC 7120 (also known as Nostoc sp. PCC 7120). The presence of two Suc-phosphate synthases (SPS), SPS-A and SPS-B, directly involved in Suc synthesis with different glucosyl donor specificity, seems to be important in the N 2 -fixing filament. Measurement of enzyme activity and polypeptide levels plus reverse transcription-polymerase chain reaction experiments showed that total SPS expression is greater in cells grown in N 2 versus combined N conditions. Only SPS-B, however, was seen to be active in the heterocyst, as confirmed by analysis of green fluorescent protein reporters. SPS-B gene expression is likely controlled at the transcriptional initiation level, probably in relation to a global N regulator. Metabolic control analysis indicated that the metabolism of glycogen and Suc is likely interconnected in N 2 -fixing filaments. These findings suggest that N 2 fixation may be spatially compatible with Suc synthesis and support the role of the disaccharide as an intermediate in the reduced C flux in heterocystforming cyanobacteria.