A disease like physiological disorder of hazelnut known as "brown spots" was thoroughly studied for the first time from the ItaJian hazelnut growing areas. The quali-quantitative composition of the bacterial microflora associated to the syndrome was investigated. Six bacterial species, belonging to four different families were constantly isolated from the syndrome. On the basis of traditional and molecular assays, the species associated to brown spots were identified as Pantoea agglomerans, Microbacterium schleiferi, Xanthomonas arboricola pv. corylina, Pseudomonas fulva, Pseudomonas putida and Brenneria alni. Quantitative differences, statistically significant, were observed among the species. The populations, expressed as colony forming units per gram of the sample (CFU/g), ranged from a lowest value of 3.6xJ03 to a highest value of 2.7xl04 respectively for Microbacterium schleiferi and Xanthomonas arboricola pv. corylina. Plant stress, caused by the adverse environmental conditions and inadequate agronomie and cultural practices, was the main cause of the brown spots during which the ubiquitous bacteria rapidly colonize and multiply the weakened plants. This phenomenon can be overcome if good agronomie and cultural practices are adopted.