Cultured Nostoc spec. from Peltigera horizontalis excreted in the light between 1% and 3.4% of fixed 14CO2 into the medium, less-than-or-equals, slant0.8% as glucose. A several-fold increase in the rate of release was caused by addition of carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) in the dark, the major component released being glucose. The amount of glucose excreted in the presence of FCCP exceeded the pool of intracellular glucose + glucose 6-phosphate by far, suggesting a glucan as a precursor. A glucan — identified enzymatically with amyloglucosidase — was the main assimilation product which was degraded during excretion of glucose in the dark in the presence of FCCP. Of the free-living cyanobacteria, Nostoc punctiforme also showed glucose release whereas Nostoc muscorum did not. It is concluded that (i) the symbiotic Nostoc spec. can excrete glucose in the absence of the fungal partner, (ii) this release is a passive process being fed by a glucan and (iii) glucose release is not a specific function of symbiotic cyanobacteria.