Cambial seasonal activity and the dynamics of xylem growth ring formation were studied during the 2006 growth season in mature beech (Fagus sylvatica) trees at a forest stand near Ljubljana, Slovenia. Cross-sections of tissues taken from living trees at weekly intervals were studied with light microscope. Cambial cell divisions began between 18 and 24 April, the cambium achieved maximum width (10-13 cell layers) at the end of May, and cell divisions ceased between 25 July and 16 August. Dormant cambium contained 3 to5 cell layers. Fitting xylem increments to the Gompertz function showed that the period of most intense cell production was from 30 May to 6 June 2006.The average width of fully formed xylem growth rings was 2552 mm, and the time necessary for their formation was 100 days. Although the investigated trees were healthy and of comparable age, the parameters of the Gompertz function showed differences in radial growth patterns among trees.