Polyamines are primordial， small organic polycations present in almost all cells， but their roles in bacteria are poorly understood. sym-Homospermidine is the dominant polyamine in the filamentous， N -fixing， heterocyst-forming cyanobacterium Anabaena sp. PCC 7120. Synthesis of homospermidine was dependent on speA (encoding arginine decarboxylase)， speB (agmatinase) and speY (deoxyhypusine synthase homologue)， which in bacteria is an unprecedented pathway. Inactivation of any of these genes impaired diazotrophic growth. Heterocyst differentiation in the speA mutant was blocked at an early step， after induction of the regulatory gene hetR but before production of heterocyst-specific glycolipids (HGL). In contrast， the speY mutant produced HGL and showed slow diazotrophic growth. Analysis of fusions to green fluorescent protein revealed that SpeA (like SpeB previously described) accumulates at higher levels in vegetative cells than in heterocysts， and that SpeY accumulates in vegetative cells but also at significant levels in heterocysts. The homospermidine biosynthetic pathway is therefore active primarily in vegetative cells but the last step can be completed in heterocysts. Our findings indicate an important role for polyamines in the diazotrophic biology of Anabaena. Furthermore， inactivation of a gene cluster (potADB) encoding a polyamine ABC transporter disrupted diazotrophic growth， corroborating the importance of polyamine homeostasis in Anabaena.