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Alkaline phosphatase and peroxidase activities were investigated in typical neutrophils and in vacuolated cells, the latter considered to be another type of neutrophil in catfish blood. This study was carried out by light- and electron-microscopy. Two lines of neutrophils were recognized in the peripheral blood of catfish. Alkaline phosphatase activity was present only in the vacuolated neutrophils; peroxidase activity appeared only in the typical neutrophils. The presence of two types of neutrophil in catfish blood suggests that important steps of neutrophil evolution occurred in fish.