Cambial activity, formation of xylem growth-ring, and differentiation of xylem cells were studied in 6 trees of common beech (Fagus sylvatica L.) from Panška reka near Ljubljana and in 6 trees from Menina planina. The sampling was performed in the second half of the growing season 2008. Micro cores were collected from living trees at weekly intervals from July 7 - September 8 (Panška reka) and from July 7 - September 12 (Menina planina). They contained wood, cambial zone and bark. Histological slices were cut with rotary microtome, and stained with safranine and astra blue stains, then they were observed under the light microscope and analyzed with image analysis system. The number of cell layers in the cambial zone was counted and the width of the forming tree-ring measured. At the beginning of the sampling, the cambial zone at Panška reka contained 9 - 10 and at Menina planina 7 - 8 layers of cells in the radial row. Cell divisions at Panška reka were completed from July 28 - August 25 with the exception of 1 tree (division completed by July 7) and at Menina planina from 7th - 18th July. The lignification of the fibre cell walls at Panška reka was completed from August 11 - September 8, and at Menina planina from 8th - 22nd August, 2008. The dormant cambial zone contained 3 - 5 layers of cells in radial row.