Vegetative mycelia of Pleurotus ostreatus were differentiated into primordia and subsequently into fruit bodies in synthetic sucrose-asparagine medium when exposed to light at low temperature. During photomorphogenesis, L-ascorbic acid-like substances called reductones were produced. L-Ascorbic acid, D-erythroascorbic acid, 5-O-(${alpha}$-D-glucopyranosyl)-D-erythroascorbic acid, 5-O-(${alpha}$-D-xylopyranosyl)-D-erythroascorbic acid, 5-methyl-5-O-(${alpha}$-D-glucopyranosyl)-D-erythroascorbic acid and 5-methyl-5-O-(${alpha}$-D-xylopyranosyl)-D-erythroascorbic acid were accumulated initially in the illuminated mycelia before the initiation of fruiting. The content of glycosides of erythroascorbic acid and their methylated compounds increased again in the primordia and the fruit bodies. Exogenous L-ascorbic acid induced the formation of primordia from the mycelia in the dark in a dose-dependent manner. Thus, this suggests that these reductones might play a role in mediating the light stimulus in photomorphogenesis.