The purpose of our research was to investigate influence of ecological factors on regeneration occurrence with special references to its structural characteristics and dynamics. We studied mixed old-growth Fagus-Abies-Picea forest reserve of Lom in the Dinaric Mountains of Bosnia and Herzegovina. Forest is characterized by low densities of ungulates and less air pollution in the past. We carried out the investigation in 2010 when the grid of 150 research plots was established on a surface of 15 ha. On each plot the coverage of tree regeneration and ground vegetation, regeneration density in different height classes were recorded with special emphasis on the spatial pattern of its occurrence. Relative diffuse and direct radiation was estimated using digital hemispherical photographs. Average direct radiation value was 5,5 % and diffuse radiation value 5,4 %. The average total one-year-old seedling density in Lom was 2,2 m-2 and regeneration density 2,0 m-2, respectively. There was a significant relationship between coarse woody debris coverage and occurrence of conifer one-year-old seedlings, and smaller spruce seedlings. The abundance of fir regeneration was the highest within the small seedling class (h < 20 cm) amounting 76 %. The amount of fir decreased with increasing tree height. However, in the height class 20-250 cm, the beech seedlings with 72 % dominated. The highest density of fir one-year-old seedlings and smaller beech seedlings appeared on micro-sites, where higher values of direct radiation are present. Analyses suggested that fir regenerates continuously and recruits successfully into larger height classes which ensures the stability of forest stand and confirms the assumption of the high dynamics in fir-beech forest.