This study demonstrates that the fish pathogenicAphanomyces piscicida (=A.invadans) possesses hemagglutinating and hemolytic activities. These activities were observed against goldfish erythrocytes using supernatant of the homogenized hyphae ofA. piscicida. The hemagglutinating activity was not inactivated by heat treatment at 60°C for 30 min or by the addition of EDTA. D-galactose or lactose inhibited the hemagglutinating activity. D-galactose-binding substances which were isolated by affinity chromatography showed hemagglutinating activity. On the other hand, heat treatment at 60°C for 30 min reduced the hemolytic activity. It was demonstrated that a sample that did not absorb to D-galactose in affinity chromatography was responsible for the hemolysis. The factors that have hemagglutinating and hemolytic capacities may affect the blood cells of infected fishin vivo, and may play a role in the occurrence of anemia in infected fish.