Nostoc punctiforme is the microsymbiont of the unique fungal endocyanosis Geosiphon pyriforme. During the establishment of this endosymbiosis fungal hyphae incorporate a specific stage of the N. punctiforme life cycle, formed during differentiation from the motile hormogonium to the non-motile primordium stage. The life cycle of N. punctiforme was synchronized by first inducing hormogonium differentiation by red light illumination for 36 h, followed by applying light/dark cycles for 96 h. Different developmental stages of N. punctiforme including hormogonia, primordia and vegetative colonies were analysed by fluorescein isothiocyanate (FITC)-conjugated lectins with sugar specificity to α-D-mannose/α-D-glucose, N-acetyl-β-D-glucosamine oligomers, α-L-fucose, β-D-galactose, α-D-galactose, N-acetyl-α-D-galactosamine and sialic acid. Lectin binding properties indicate that the slime produced by the motile hormogonia contains β-D-galactosyl and α-L-fucosyl residues, which disappear during further development. There is a clear correlation between the transition from the hormogonium to the non-motile primordium stage and the appearance of a large amount of α-D-mannosyl (or α-D-glucosyl) residues in the extracellular slime. This strongly Concanavalin A (Con A)-positive compound is lost in part during later differentiation into vegetative colonies. Consequently these significant changes in the Nostoc slime, occurring at the developmental stage involved in the establishment of the Geosiphon symbiosis, could play a role in the specific recognition between the symbiosis partners.