The legume inoculant Rhizobium leguminosarum was grown on peat in roller bottles and a rotating drum bioreactor. Growth conditions were first determined using experiments in roller bottles. The best growth rate and cell count were obtained with peat containing 40% moisture (wet basis) and a volume fraction of moist peat of 0.3 in the roller bottle bioreactors. A cell count of 1.3 to 1.9 x 10(9) CFU/g was achieved in 4 d or less, from an initial inoculum of 10(6) CFU/g. in a rotating drum bioreactor, the growth of R. leguminosarum on peat with sucrose as the main carbon source was completed after 4 d. The majority of growth was in the first 2 d, based on carbon dioxide evolution. These results showed that growth of R. leguminosarum was more rapid in the rotating drum than in bags of peat, due to enhanced oxygen transfer.