We investigated the seasonal activity of the cambial zone and the dynamics of xylem growth-ring formation in the 2006 growth period in six beech (Fagus sylvatica L.) trees from Panjska reka near Ljubljana. We removed intact tissue samples of wood, cambial zone and bark from living trees at weekly intervals during the period from 24 April to 22 August 2006. Histological sections were prepared on a rotary microtome and stained with safranine and astra blue. Analyses were done with a light microscope and an image analysis system. We described the phases of differentiation of xylem elements (vessels, fibres and axial parenchyma). On microscopic slides of tissues, we counted the number of cell layers in the cambial zone and measured the width of the cambial zone and current xylem increment. The dormant cambial zone contained 3 to 5 layers of cells. In all trees, cell divisions in the cambial zone began between 18 and 24 April 2006. The first cells in postcambial growth were observed on 18 April 2006, while secondary wall formation began a week later. The cambial zone achieved maximum width at the end of May, when the cambial zone contained 10 to 13 cell layers. We observed completely differentiated fibres, with fully formed cell walls in the week between 13 and 20 June 2006. Divisional activity in the cambial zone ceased between 25 July and 16 August 2006. The annual ring was completely formed at the end of August. The measured values of xylem increments fitted the Gompertz function with an agreement of 98.7 %. The period of most intense cell production in the cambial zone was from 30 May to 6 June 2006. The average width of fully formed xylem growth-ring was 2537.68 m, and the time necessary for its formation was 100 days. We observed differences among trees in terms of the dynamics of xylem growth-ring formation and its final width.