Eubacterium limosum KIST612 was cultivated on carbon monoxide using a cell-recycled continuous fermentation system to produce organic acids. A bubble column reactor system (k(L)a = 72 h(-1)) was used with a membrane to allow on-line products removal with cell recycle. When cell concentration reached 5.25 g l(-1) in the reactor, the cell concentration did not increase at carbon monoxide (CO) partial pressures lower than 74 kPa though CO was consumed with the acidic products formation. At this stage, the overall CO mass transfer rate (k(L)a[C* - C-L]) was lower than required for the maximum cell growth (q(CO)(X)(max) but higher than that to meet the maintenance requirement (m(s)X). When the CO mass transfer rate was maintained higher than maintenance requirement by increasing CO partial pressure, the cell concentration increased to 9.5 g l(-1).