² Hyogo Prefectural Fisheries Experimental Station, Futami, Akashi, Hyogo 67 4-0093, Japan

A marine algicidal gliding bacterium Cytophaga sp. strain J18/M01 was isolated using the harmful red tide alga Chattonella antiqua as a susceptible organism from a station in northern Harima-Nada, the Seto Inland Sea, Japan, in 1990. The bacterium can prey upon various species of microalgae. Temporal fluctuations of this bacterium and Chattonella spp. (C. antiqua and C. marina) were investigated weekly at the above station in the summer of 1997 and 1998, with the immunofluorescent assay employing polyclonal antibodies for the bacterium Cytophaga sp. strain J18/M01. This polyclonal antibody showed the highly specific reactivity, and other several phylogenetically close bacteria did not react with this antibody. In the summer of 1997, the cell density of Chattonella spp. showed a maximum value (70 cells / ml) on 8 July, an d decreased thereafter. The bacterium Cytophaga sp. was commonly detected a round a few hundreds cells / ml or less. The number of Cytophaga sp. increased after the peak of Chattonella spp. and the maximum cell number of the bacterium was 1300 / ml. This algicidal bacterium also followed the changes of total amounts of microalgal biomass (chlorophyll-a + pheophytin) when Chattonella spp. were absent. In the summer of 1998, Chattonella spp. were scarce, and the algicidal bacterium Cytophaga sp. showed a close relationship with the change of total microalgal biomass. The present results suggested that the algicidal bacterium Cytophaga sp. preyed upon not only harmful red tide microalgae but also common microalgae such as diatoms when they are blooming, and the bacterium presumably play an important role in regulating microalgal biomass within some homoeostatic level in the coastal sea.