Abstract: Erythrocyte osmotic fragility and erythrocyte membrane fatty acid profiles of Rainbow trout (Oncorhynchus mykiss) experimentally infected with Aeromonas salmonicida were investigated at different days of injection. Erythrocytes of the infected fish were found more fragile than the controls. While 50% of haemolysis of diseased fish erythrocytes occurred at chloride concentration of 0.49±0.01%, it was 0.40±0.01% for controls. Three remarkable trends in erythrocyte fatty acid patterns in infected fish observed when compared with the controls. While no significant differences in saturated fatty acids were detected significant decrease in monounsaturated acids and increase in polyunsaturated fatty acids were also detected.