S U M M A R YPerithecial initiation is strongly inhibited by visible light in less than 18-hr-old hyphae of Sordariajimicola which are grown in the presence of I p~-6-thioguanine. It is known that 6-thioguanine is destroyed when exposed to light in vitro with oxygen and a sensitizing dye like methylene blue is present. The most effective wavelength region for perithecial inhibition is the blue region, indicating that possibly flavins are the photosensitizing compounds in this case. The synthesis of carotenoid and melanin pigments is also affected by blue light in this organism. Another, transient, inhibitory effect on perithecial production is found when light-grown cultures are transferred to darkness. As a consequence of the two light effects, mycelia exposed to alternating light and dark periods in growth tubes develop welldefined zones of perithecia. A simple state-input model is presented in the Appendix to demonstrate how such zonation patterns can be generated on the basis of the light effects and a few other assumptions.