Fig. 1. Relationship between UV intensity and irradiation period for complete suppression of reactivated flagella. UV irradiation covered the entire flagellum and flagellar movement stopped completely within this period. The intensity of UV irradiation was defined as a relative value. Irradiation without the ND filter was defined as 100% (see Materials and methods). The concentration of PRODAN was 0.02 µmol l^–1 (diamonds), 1 µmol l^–1 (squares), 5 µmol l^–1 (triangles) and 10 µmol l^–1 (crosses).