We found that the red tide dinoflagellate Karenia mikimotoi (Gymnodinium mikimotoi) generates reactive oxygen species (ROS). In chemiluminescence analysis using an O-2(-) specific probe, a slightly lower but significant level of O-2(-) was detected in a K. mikimotoi cell suspension as compared to one containing Chattonella marina, a well-known ROS-producing red tide phytoplankton. Interestingly, the chemiluminescence response pattern caused by K. mikimotoi was different from that of C marina, although both responses were almost completely abolished in the presence of superoxide dismutase (SOD). By fluorescence spectrophotometric analysis, H2O2 was also detected in the K. mikimotoi cell suspension, but the level of H2O2 was approximately 10% of that in the C marina suspension based on the cell number. No significant increase in O-2(-) generation by K. mikimotoi was observed in the presence of lectins such as concanavalin A (Con A) and wheat germ agglutinin (WGA) or fish mucus substances prepared from the skin and gills of yellowtail, whereas C marina generated an increased level of O-2(-) in response to these stimuli. Furthermore, O-2(-) production in C marina, but not in K. mikimotoi, was inhibited by treatment with proteinase K.