Several biochemical properties of a mutant deficient in phosphoglucose isomerase (pgi) ofCoprinus cinereuswere examined in connection with its ability to produce basidiocarps. Mycelium of thepgimutant accumulated glucose-6-phosphate and showed higher glucose-6-phosphate dehydrogenase activity than wild type. A conventional fruiting medium did not support basidiocarp formation by the homozygous dikaryonpgi/pgi, but the addition of a reduced amount of glucose plus a compensatory amount of fructose to the culture medium resulted in partial recovery of fruiting ability. This modification of the culture medium decreased the intracellular glucose-6-phosphate to almost the same level as in the wild type. The addition of polyols such as mannitol resulted in complete recovery of fruiting ability by the dikaryonpgi/pgiwithout affecting the level of glucose-6-phosphate. Mycelium of the mutant showed an elevated activity of NAD-linked polyol dehydrogenase and an elevated intracellular NAD level, irrespective of whether the mycelium was grown in the presence or absence of polyol.