ABSTRACT:The effect of thinning of young spruce stands by 500 trees/ha on biological activity in the soil profile was studied in the mountainous area of the Moravian-Silesian Beskids. The biological activity of soil was determined under optimal laboratory conditions by tests of soil respiration, catalase activity and intensity of cellulose decomposition. No statistically significant differences were found between the individual biological tests when the two experimental stands were compared (P-level 0.05). All biological activities within each stand were correlated, and significant correlations were found between biological activities in the soil and ammonium nitrogen content.