SUMMARY Aspergillus ornatus produces conidia when grown in continuous light but few, if any, when grown in continuous darkness. A minimum of 3 h of exposure to light is needed for induction. Light inhibits growth, glucose uptake and phosphoryl- ation but does not inhibit the uptake of lysine. A low molecular weight substance produced or accumulating in the light inhibits the phosphorylation of glucose. It is suggested that the inhibition of glucose uptake and phosphorylation precedes conidiation and that conidiation may be the result of starvation caused by this light-induced inhibition.