Four kinds of Nicotiana species and five varieties belonging to N. tabacum were used as materials for electrophoretic analysis of the tetrazolium oxidase isozyme to examine the taxonomic affinity among them based on the biochemical property. All the five verieties of N. tabacum showed same isozyme bands despite the fact that these varieties had notably varied characteristics including morphological traits. The band patterns were more or less different among the four species. Although N. rustica and N. tabacum were of the same genome group of AABB, their isozyme bands showed considerable difference. N. sylvestris, genome A donor of Nicotiana species, was found to be markedly different from N. tatacum in band pattern, including the absence of system 2 in N. sylvestris.