Abstract: In the framework of the EUFORGEN project, an experimental field of 26 provenances ofcork oak from several Mediterranean countries was set up since 1997 in two sites (Tebaba andHanya) located in the North-western Tunisia and characterized by humid bioclimate. Until 2005the plants in the site of Tebaba didn’t show any symptom of weakness or decay. However, inSeptember 2006 different decay degrees of the provenances were registred in the same site.In order to evaluate the damage severity, four classes of crown dieback (for 1/3, 1/2, 2/3, andwhole crown) were considered. The results showed that 24% of the present trees (2340) areaffected, and 87% of these completely died. The provenance of Makna (Tunisia) appeared to bethe most sensitive, while that of Fuencaliente (Spain) was the least affected. The analyses carriedout on different tree parts (shoots, branches, stem, roots) showed the presence of variouspathogenic agents, some of which seem to be involved in aetiology of the tree decline, i.e.Biscogniauxia mediterranea, Discula quercina, Hypoxylon sp., and Phomopsis sp. However, nosoil pathogens were found.