SUMMARYThe inhibitory effects of light on the development ofGonatobotryum fuscumon its host,Graphiumsp., has been attributed to the destruction by light of vitamin B6, which is required for growth and other metabolic activities of the host. Pre-inoculation exposure of 4 days at 300 ft-c of white light rendered the medium unsatisfactory for growth of the parasite on its host, even in darkness, unless Bo was again added. A higher concentration of B6is required for development of the parasite than for growth of the host. However, the addition of biotin, which has no visible effect on growth ofGraphiumsp., allows the development of the parasite on its host, even on media having a low concentration of B6. The conclusion is reached that biotin is essential for the synthesis by the host of a nutrient required by the parasite. UsingSordaria fimicolaas an assay fungus, it was shown that the amount of biotin synthesized byGraphiumsp. was correlated with the amount of B6in the medium. Evidence is presented thatG. fuscumsynthesizes and excretes vitamin B6.