Previously, a chloroplast-localized Cu,Zn superoxide dismutase (chCu,ZnSOD) was isolated from lily and the sense- and antisense- sequences of the lily chCu,ZnSOD were used to transform potato plants. Two selected lines, the sense- and anti-sense strand of transgenic plants, were further characterized for resistance to Erwinia carotovora, which is a severe pathogen affecting potato plants. Only the sense-strand transgenic potato, which contained less O2 .- and more H2O2 than wild-type and antisense-strand transgenic plants, showed increased resistance to E. carotovora. Additional studies using O2 .- or H2O2 scavengers in wild-type, sense-strand, and antisense-strand transgenic plants suggest that resistance to E. carotovora is induced by reduced O2 .- and is not influenced by H2O2. To the best of our knowledge, this report is the first study suggesting that resistance to E. carotovora is enhanced by reduced O2 .-, and not by increased amounts of H2O2.