To examine the effects of ozone (O$_3$), one of the major air pollutants in the city area, on genetic changes in Cornus controversa Hemsl., we compared genetic structures between sensitive (S) and tolerant (T) tree groups of C. controversa fumigated with ozone using isozyme markers. The genetic structures were measured in terms of allele and genotype frequencies determined at ave polymorphic enzyme loci. Marked genetic differences between the two groups were detected at three loci (Lap-2, Mdh-1 and Skdh-1). Genetic parameters, genetic multiplicity, genetic diversity and heterozygosity showed that the tolerant group retained greater genetic variation than did the sensitive group. Results of the study were congruent with the general expectation that the more heterozygous individuals and/or populations exhibit higher resistance to various stress factors.