The virulence of 44 isolates of Thielaviopsis basicola collected from tobacco fields in Japan was examined by inoculating them to seedlings of susceptible cv. BY4 and weakly resistant cvs. MC1 and BurIey 21, which are all commercial cultivars in Japan. Average disease indices (DI, 0: healthy - 5: complete root rot) of BY4 were always over 3.5 but those of MC1 and Burley 21 ranged from 0.8 to 4.3. Virulence to resistant cultivars was tentatively classified into three groups: highly virulent (DI3), moderately virulent (DI: 2-3) and weakly virulent (DI<2). Nine isolates (three for each group) were inoculated to five resistant cultivars having different genetic backgrounds. The average disease indices of cvs. Delcrest and Hicks Broadleaf were almost the same as those of MC1 and Burley 21 inoculated with all the isolates. The average disease indices of cvs. Riwaka 5, NFT 706 and Virginia Gold were 4.2, 2.5 and 4.2 respectively, when inoculated with highly virulent isolates, and were less than 1.0 when inoculated with moderately or weakly virulent isolates. In order to determine highly virulent isolates which might cause economically important damage, their inoculation to resistant cv. Riwaka 5 and susceptible cv. BY4 will be recommended.