Bradyrhizobium japonicum, a nitrogen-fixing bacterium in soil, establishes a symbiotic relationship with the leguminous soybean plant. Despite a mutualistic association between the two partners, the host plant produces an oxidative burst to protect itself from the invasion of rhizobial cells. We investigated the effects of H(2)O(2)-mediated oxidative stress on B. japonicum gene expression in both prolonged exposure (PE) and fulminant shock (FS) conditions. In total, 439 and 650 genes were differentially expressed for the PE and FS conditions, respectively, at a twofold cut-off with q < 0.05. A number of genes within the transport and binding proteins category were upregulated during PE and a majority of those genes are involved in ABC transporter systems. Many genes encoding ? factors, global stress response proteins, the FixK(2) transcription factor, and its regulatory targets were found to be upregulated in the FS condition. Surprisingly, catalase and peroxidase genes which are typically expressed in other bacteria under oxidative stress were not differentially expressed in either condition. The isocitrate lyase gene (aceA) was induced by fulminant H(2)O(2) shock, as was evident at both the transcriptional and translational levels. Interestingly, there was no significant effect of H(2)O(2) on exopolysaccharide production at the given experimental conditions.