In raphydophyte cell suspensions such as those of Chattonella marina, C. ovata and Heterosigma akashiwo, a gradual increase in NO-specific fluorescence intensity was observed, and the increase in the fluorescence intensity of each of these phytoplankters was completely inhibited in the presence of carboxy-PTIO, a specific NO scavenger. However, no such significant changes were observed in the case of other phytoplankter species.