The advantages of plant growth promoting rhizobacteria in an Advanced Life Support system on the Moon or Mars may be limited by a low partial pressure of oxygen (pO2). Raphanus sativus ‘Cherry Bomb II Hybrid’ was inoculated with Bradyrhizobium japonicum Tal 629 and grown hydroponically at ambient and low (7 kPa) pO2 for 18 days. Inoculation with B. japonicum was deleterious to radish productivity (22% reduction in dry weight overall) and this was slightly exacerbated by low pO2. The microbial load of the hydroponics system was not affected by inoculation, though it was reduced by low pO2. Microbial abundance declined after the first 5 days of chamber closure with no further changes after the next 10 days. Microbial community composition changed significantly with inoculation.