Microflora in the digestive tract of yellow catfish (Pseudobagrus fulvidraco) and in the water were studied in this paper.Results showed that:the logarithm of aerobic and facultive,anaerobic bacteria of foregut,midgut and hindgut were 5.27±0.74,6.26±0.36,7.56±0.61,respectively.Obligate anaerobic bacteria numbers were 2 to 3 orders of magnitude more than the other two kinds with Bifidobacterium the dominant species.The logarithm of bacteria in water was 4.80±0.73.It indicated that the concentration of nutrients were not very high.Aeromonas,Enterobacteriaceae and Acinetobacter were dominant species in the digestive tract of fish.They were also dominant species in water.In addition,the number of bacteria in the digestive tract of Pseudobagrus fulvidraco suffered from hemorrhage disease significantly increased.Aeromonas increased 18.3%~33.6%.And bacteria species decreased significantly.It showed that the opportunistic pathogens can cause diseases only in certain conditions and based on quantitative superiority.