Samples of wood, bark and cambium from six adult beech trees (Fagus sylvatica L.) were collected at weekly intervals throughout the 2006 growing season. The selected sampling trees originated from the forest at Panška reka, near Ljubljana. The tissue cross-sections were observed under the light microscope.On the tissues taken from April until September 2006 the changing width of cambial zone and the formation of the tree ring in wood and in phloem were recorded. In the trees No. 5 and No. 6 the first formed cells occurred in the wood between 18 and 24 April 2006. This coincided with general leaf unfolding of the trees. First mature cells, with completed radial growth and cell wall lignification, were observed 8 weeks after their formation in the cambium. On 20 June it was possible to clearly differentiate early and late wood. Formation of new cells of wood ended in late July or in early August. The development of all cells was completed at the end of August.The dormant cambial zone contained 3 to 5 cells in a radial row. At thepeak of the cambial activity (between 25 April and 2 May 2006) the cambial zone consisted of 10 to 13 cells. The development of phloem ring could be observed on cross sections as well. Finally, the importance of the obtained results for forestry practice is presented.