Spawned yellow catfish (Pseudobagrus fulvidraco Richardson) and their fry usually die of infective diseases. In order to enhance the resistant ability of spawning yellow catfish and their fry against pathogens, it is necessary to illuminate changes of immune parameters of periphery blood in immunized mature yellow catfish. In this study, one hundred and thirty healthy mature yellow catfish were randomly divided into test and control groups. Each individual was intraperitoneally injected with 0.2 mL of bacterial suspension at a concentration of 1.0×10⁸ CFU/mL formalinkilled Aeromonas sobria for the test group, and with 0.2 mL of 0.65% sterile saline solution for the control group. Then, approximate 2 mL of blood was taken from each fish caudal vein at 1, 2, 4, 7, 14, 21, 28, 35 d post injection. Three yellow catfish were sacrificed at each sampling time. The haematological indices, such as leucocyte and erythrocyte number per microlitre, differential leucocyte count (DLC), and serum antibody titer and activity of phagocytes, were investigated for each blood sample of the test and control group. The results were as follows : The leucocyte number per microlitre of test group drastically increased and was significantly (or very significantly) higher than that of control from the 2nd day to 35th day post immunization (Fig. 2). The differential leucocyte count (DLC) of the test group prominently increased and was significantly (or very significantly) higher than that of control for neutrophil and monocyte from day 4 to day 7 (Fig.5 & Fig.6 ), and for lymphocytes from the 4th day to 28th day (Fig. 4). The phagocytic percentage and phagocytic index also gradually increased from day 1 to day 7 and peaked at 7 d, then dropped down to the control level (Tab. 2). However, the test group was significantly lower than the control in erythrocyte number per microlitre between the 7th day and 35th day (Fig. 1), and in DLC of thrombocyte between day 4 and day 28 (Fig. 3). The serum antibody titer gradually increased from day 1 to day 14 and reached the highest value at 14 d, then, slowly decreased from day 14 to day 35, and maintained a higher level between day 14 and day 21 in comparison with the values at other sampling times (Tab. 3). From what has been stated above, the immunized mature yellow catfish were enhanced in their resistant ability against bacterial pathogens by means of increment of nonspecific cellular immunity, such as monocyte and neutrophil, during the early period (from day 1 to day 7) post single dose immunization, and of specific cellular and humoral immunity, such as lymphocytes and antibody, from day 7 to day 35 after the immunization. And the higher antibody titer level could last for a week from day 14 to day 21 post the immunization. Therefore, it is suggested that the mature fish should be immunized with singledose vaccine at day 10-14 before breeding to enhance the resistant ability of spawn yellow catfish against pathogens, and further to reduce mortality rate of the spawned yellow catfish and their fry caused by infective diseases.