Olisthodiscus sp. is classified in the class Raphidophycea of the division Chrysophyta. Other raphidophytes, Heterosigma spp. and Chattonella spp., are well known as the causative organisms of red tides, and are highly toxic to fish. However, the toxic potential of Olisthodiscus spp. has not been established. Recent studies demonstrated that Heterasigma spp. and Chattonella spp. generate reactive oxygen species (ROS) such as O-2(-) and H2O2 In this study, we found that Olisthodiscus luteus also produces O-2(-) and H2O2 under normal growth conditions. The levels of O-2(-) and H2O2 produced by O.luteus were comparable to those of Heterosigma akashiwo. We therefore speculated that O.luteus also exerts ROS-mediated harmful effects on surrounding living organisms. In fact, O.luteus inhibited the proliferation of marine bacteria, Vibrio alginolyticus, in a plankton/bacteria co-culture system. Furthermore, the toxic effect of O.luteus on V.alginolyticus was suppressed by the addition of superoxide dismutase (SOD) and catalase. Although the detailed mechanism of the production of ROS by O.luteus is still unclear, O-2(-) production was increased by treatment with lectins such as wheat germ agglutinin (WGA), castor bean hemagglutinin (CBH) and concanavalin A. (Con A), suggesting that cell surface redox enzymes may be responsible for the production of ROS. This hypothesis was supported by the finding that proteinase K inhibited O-2(-) generation by O.leteus. In conclusion, our results suggest that O.luteus exhibits ROS-mediated harmful effects on surrounding living organisms.