The beginning of seasonal activity in the cambial zone of beech (Fagus sylvatica L.) was investigated on 2 forest sites: Panška reka (400 m a.s.l.) and Menina planina (1,200 m a.s.l.). The beginning of cell differentiations and early wood formation in xylem tree-ring 2008 were observed. From each forest site, micro samples of wood, cambial zone and bark were removed from 6 living trees at weekly intervals, from March 17 to June 23 (Panška reka), and from April 18 to July 18 (Menina planina). Histological sections were prepared on a rotary microtome. They were stained with safranine and astra blue and analysed with a light microscope and with an image analysis system. On microscopic slides of tissues the cells in cambial zone were counted, and the widths of current xylem increment measured. The dormant cambial zone contained 4-5 layers of cells. In Panška reka the first divisions in the cambial zone began from April 14-24, on Menina planina from May 9-16. On both sites the first cells in post cambial growth were observed 1 week after activation of cambial zone, while secondary wall formation began 2 weeks later. The first completely differentiated fibres with completely formed cell walls were observed on both plots 5-6 weeks after activation of cambial zone. The widest cambial zone (10-14 cell layers) on Panška reka was observed from June 2-16. On Menina planina the cambial zone was constant after activation; it was from 8-10 cell layers wide, measured in radial direction. At the end of experiment no cells of late wood were tracked. Differences in wood formation dynamics and width of xylem tree-rings were found among trees and forest sites.