Among many Heterosigma akashiwo-killing bacteria isolated from Hiroshima Bay in 1994 and 1995, an Alteromonas sp. strain GY9501 is considered to dominantly influence rapid termination of H. akashiwo bloom .GY9501 has a high algicidal activity upon dinoflagellate Gymnodinium mikimotoi and Heterocapsa circularisquama, beside H. akashiwo. A result of an experiment using two component cultivation system suggested that GY9501 kills the algae through unknown algicidal substances. Strong algicidal activity against H. akashiwo was observed in GY9501-H. akashiwo co-culture filtrate. After ultrafiltration of the filtrate, H. akashiwo-killing substance (HAK) existed in 3,000-10,000 MW fraction (HAK-1) and less than 1,000 MW fraction (HAK-2). HAK-1 is heat-stable and has weak algicidal activity on G. mikimotoi, H. circularisquama, and other four red tide causing algae. HAK-1 didn\'t exist in GY9501 monoclone culture in a pepton medium nor in a H. akashiwo culture filtrate including extracellular organic matter.