Mycosporin I yield of Nectria galligena Bres. in response to duration, intensity, and spectral quality of illumination were evaluated by high-performance liquid chromatography (HPLC). Duration and nature of irradiation were essential light factors for an increase of mycosporin content in the mycelium. The most effective wavelengths were 320, 370, 420, 450, and 500 nm. In addition, it was demonstrated that mycosporin I and ascocarp production required very similar light conditions, and the rate of sexual reproduction as well as mycosporin I yield were correlated. Furthermore, photomorpbiogenic action of mycosporin I extracted from N. galligena was demonstrated. This substance, when added to culture broth, served as a partial substitute for the active light and induced fertile ascocarps in darkness. Moreover, it was shown that, while mycosporin I (related to reduced serine) characteristic of N. galligena was the most active compound, the type II mycosporins (related to reduced glutamine and glutamic acid) were also morphogenic effective. Because of its photodependent biosynthesis and its photomimetic (morphogenic and metabolic) activities, mycosporin I appears to be an outstanding mean for the the study of the sexual photoinduction in N. galligena.