Alexandrium tamarense is a notorious harmful algal bloom species,which is associated with the largest number of paralytic shellfish poisoning cases,causing devastating economic losses and health hazards.Mangrovimonas yunxiaonensis strain LY01 which produced algicidal compound showed high algicidal activity on A.tamarense.Transmission electron microscope analysis revealed that the cell membrane was broken,and vacuoles and chloroplast abnormalities showed up.