Sinorhizobium meliloti possesses three distinct catalases to cope with oxidative stress: two monofunctional catalases (KatA and KatC) and one bifunctional catalase-peroxydase (KatB). The katB gene is constitutively expressed during growth in batch culture and is not induced under oxidative stress conditions. In contrast, the expression of katA and katC genes is mainly regulated at the transcription level in these conditions. A differential expression of kat genes was observed during the development of the nodule. A high expression of katA gene was detected in bacteroids, suggesting that the nitrogen-fixation process induces a strong oxidative stress. In contrast, bacteria express katB and katC genes and not the H2O2-inducible katA gene in infection threads despite the detection of H2O2 around the bacteria. A katB katC double mutant nodulated poorly and displayed abnormal infection. After nonefficient release into plant cells, bacteria failed to differentiate into bacteroids and rapidly underwent senescence. Our results indicate that these two catalases are essential for the establishment of the symbiosis. They also suggest that the bacteria are in a nonexponential growth phase in infection threads and corroborate previous studies on the growth rate of bacteria inside the plant.