<it>Olisthodiscus</it> sp. is classified in the class Raphidophycea of the division Chrysophyta. Other raphidophytes,<it>Heterosigma</it> spp. and <it>Chattonella</it> spp., are well known as the causative organisms of red tides, and are highly toxic to fish. However, the toxic potential of<it>Olisthodiscus</it> spp. has not been established. Recent studies demonstrated that <it>Heterosigma</it> spp. and<it>Chattonella</it> spp. generate reactive oxygen species (ROS) such as O2- and H2O2. In this study, we found that<it>Olisthodiscus luteus</it> also produces O2- and H2O2 under normal growth conditions. The levels of O2- and H2O2 produced by<it>O.luteus</it> were comparable to those of<it>Heterosigma akashiwo</it>. We therefore speculated that<it>O.luteus</it> also exerts ROS-mediated harmful effects on surrounding living organisms. In fact, <it>O.luteus</it> inhibited the proliferation of marine bacteria, <it>Vibrio alginolyticus</it>, in a plankton/bacteria co-culture system. Furthermore, the toxic effect of <it>O.luteus</it> on<it>V.alginolyticus</it> was suppressed by the addition of superoxide dismutase (SOD) and catalase. Although the detailed mechanism of the production of ROS by <it>O.luteus</it> is still unclear, O2- 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 O2- generation by <it>O.luteus</it>. In conclusion, our results suggest that <it>O.luteus</it> exhibits ROS-mediated harmful effects on surrounding living organisms.