The differentiation and metabolism in the soil-borne saprophytic deuteromycete Trichoderma viride are subject to control by light. We have investigated the effect of illumination of mycelia on the activities of cell-wall synthesizing enzymes beta-1,3-glucan synthase and chitin synthase and on the composition of cell alls. After 10 min illumination of dark-grown mycelia with white fluorescent light at 600 1x, a gradual rise in specific activity of membrane-bound enzymes beta-1,3-glucan synthase by about 130% and a decrease in specific activity of chitin synthase by about 50% in relation to the dark control were observed. The changes in enzyme activities were caused by de novo synthesis of corresponding polysaccharide synthases(s) and/or their regulatory components since they were not observed when protein synthesis was blocked with 50 microg/ml cycloheximide. The content of beta-1,3-glucan in the cell walls of illuminated mycelia has increased by 35-50% in comparison to the dark control while the content of chitin remained practically unchanged.