We have studied two epizootic outbreaks occurred in March, 1983 and April, 1984, in fingerling rainbow trout of about 4 cm length reared in freshwater at North-West of Spain. A total of 84 strains were isolated from peritoneal cavity, liver, kidney and gills of diseased fish and were assigned to five main bacterial groups : Aeromonas, Flavobacterium-Cytophaga, Pseudomonas-Xanthomonas, Gram positive Cocci and Enterobacteria. All these bacterial groups were present in the peritoneal cavity, kidney and gills, whereas Pseudomonas-Xanthomonas and Gram (+) Cocci were not recovered from liver. Aeromonas (50%) and Flavobacterium-Cytophaga (15.6%) were the prevalent groups in the peritoneal cavity, being also detected in high percentage in kidney and gills. The majority of the Aeromonas strains belonged to the motile Aeromonas group, and were further classified to the species level.Most of the Aeromonas strains resulted to be pathogenic for rainbow trout. Four representative strains were tested for LD50 values and included in the “virulent” (LD50≤104.5) or “weakly virulent” (LD50 = 105.5) categories. These strains produced gas from glucose, were VP positive and fermented salicine, being assigned to the A. hydrophila species, and three of them shared the characteristics of the ideal phenotype of A. hydrophila biovar hydrophila displaying a strong elastase and staphylolytic activities.The analysis of physico-chemical and bacteriological parameters of the water, revealed that both epizootics appeared to be “stress” mediated since the mortalities occurred when the temperature of water suddenly increased from 5.5-8°C to more than 11°C. Then, our data confirm that motile Aeromonas septicemia is usually enhanced by elevations of water temperature.