We have isolated two superoxide dismutase cDNA clones (RsCu/ZnSod and RsFeSod) from small radish (Raphanus sativus L.) by cDNA library screening. RsCu/ZnSod is 563 bp long, with an open reading frame of 153 amino acids, and corresponds to a protein of predicted molecular mass 15.1 kDa and a pi of 5.44. The 823-bp RsFeSod has an ORF of 213 amino acids, corresponding to a protein of predicted molecular mass 25.4 kDa and a pl of 8.77. Their nucleotide and deduced amino acid sequences show the highest homology with those of Arabidopsis. Genomic Southern blot analysis, using each cDNA clone as probe, has revealed that the SOD genes are present as at least two copies in the small radish genome. Nondenaturing poly-acrylamide gels for SOD activity has demonstrated the presence of several isozymes, depending on the organ type and developmental stage. These RsSod genes also have differential expression patterns in response to treatments with white light, xenobiotics, UV, osmoticums, plant hormones, and salicylic acid. Therefore, we suggest that they are involved in an antioxidative defense mechanism against stress induced by environmental change.