We have recently proposed the existence of a bacteriocolonic pathway for ethanol oxidation, i.e ethanol is oxidized by alcohol dehydrogenase of intestinal bacteria resulting in high intracolonic levels of reactive and toxic acetaldehyde. This study was aimed to examine aldehyde dehydrogenase (ALDH) activity, acetaldehyde consumption and production of acetate by aerobic bacteria (n=27), representing the normal human colonic flora. Most bacterial strains did not show any membrane-associated aldehyde dehydrogenase, but possessed marked cytosolic NADP+- and NAD+-dependent aldehyde dehydrogenase activity, ranging from 155 nmol of NAD(P)H produced/min/mg of protein to zero with acetaldehyde as substrate. NADP+-linked ALDH activity was significantly higher than NAD+-linked activity in most of the tested bacteria. In addition, aerobic bacteria metabolized acetaldehyde effectively in vitro and this could be inhibited by cyanamide in nearly half of the tested strains. Production of acetate from acetaldehyde ranged from 2420 nmol/109 colony-forming units to almost negligible. In conclusion, many human aerobic colonic bacteria possess significant aldehyde dehydrogenase activity and can, consequently, produce acetate from acetaldehyde in vitro at least under the partially aerobic conditions proposed to prevail on the colonic mucosal surface. Individual variation in the capability of colonic flora to remove toxic acetaldehyde may be one factor regulating intracolonic acetaldehyde levels, as well as the rate of bacteriocolonic pathway for ethanol oxidation.