Charcoal impregnated with a medium was used to culture Rhizobium leguminosarum in a periodic pressure oscillating, solid-state fermenter. Effects of pressure oscillation amplitude, the span of high pressure, and the duration of low pressure on the growth of R. leguminosarum were investigated in detail. At 30 C-degrees and 3 atm, R. leguminosarum was fermented and subsequently reached 4.9 x 10(10) cfu g(-1) substrate dry matter in about 36 h, while only 1.6 x 10(10) cfu g(-1) substrate dry matter was obtained in a conventional static tray fermenter.