Abstract: The maturation of the perithecia of the pathogenic pyrenomycete Linospora ceuthocarpa (Fr.) Lind, its spore liberation, mode of infection, and the development of symptoms on aspen leaves were investigated. The study also included the histological examination of mfected leaves and a description of the cultural characteristics of the fungus. Perithecia matured in the spring, spore liberation began in the third week of June, followed by dispersal and infection. Lesions appeared on aspen leaves by the third to fourth week of July. Fast growing semimature leaves were most susceptible.