The cyanobiont (Nostoc) of the lichen Peltigera horizontalis was isolated using a procedure of repeated grinding, differential centrifugation and partitioning in an aqueous two-phase system of dextran/polyethylene glycol. A yield of 22% was calculated on the basis of chlorophyll recovery from the thallus. Freshly isolated Nostoc differed little from that kept in prolonged culture with respect to the relative activities of a number of enzymes of carbohydrate metabolism in the soluble fraction. However, much higher specific activities of the disproportionating transglycosylase and starch phosphorylase, higher intracellular concentrations of glucose, maltooligosaccharides and glycogen and a lower rate of glucose uptake were evidentin freshly isolated Nostoc. These activities and pool sizes, as well as CO2-fixation capability, stayed roughly constant for at least 5 h after isolation; glucose release, which is characteristic for the symbiotic Nostoc, dropped sharply within 1 h. These observations suggest the transport of glucose out of the cell, rather than that theactivities of cytoplasmic enzymes may bea point of early regulation during transition of Nostoc from the lichenized to the free-livingstate.