Abstract: Effects of pH stress on cortisol and non-specific immunity of Carassius auratus gibelio were investigated.Acidity and Alkalinity of the aquaculture water were maintained at pH 6.0, 7.4, 8.5 and 9.5, where pH 7.4 was used ascontrol. pH value of the aquaculture water was adjusted by HCl (1 N) or NaOH (1 N). Fish blood was sampled at 0, 14and 30 days during the experiment. Leucocytes isolated by 55% percoll were used for determination of phagocytosis andrespiratory burst activity by flow cytometry, and the plasma was used to survey the cortisol concentration and lysozymeactivity. Fish weight was measured when pH stress protocol finished. The fish was then challenged by the bacteria Flavobacteriumcolumnare, and dead fish was recorded after 7 days of the challenge. Under the stress of pH 6.0, phagocytosisof leucocytes decreased significantly at the 14 and 30 day, but no significant differences were found in respiratoryburst activity and cortisol level. There were significant differences in lysozyme activity and growth between pH 6.0 andcontrol group. No fish died from the bacteria challenge. Under pH 8.5 and 9.5 stress, the cortisol concentration increasedsignificantly, and phagocytosis and respiratory burst activity decreased significantly at the 14 and 30 day. The challengeof bacteria resulted in fish death. The results suggested that non-specific immunity and growth of the carp was heavilyaffected by the pH 8.5 and 9.5 stress, and phagocytosis and respiratory burst activity of fish leucocytes determined byflow cytometry could assess the influence of stress on fish health.