Heterotrophic nitrification by Alcaligenesfaecalis DSM 30030 was not restricted to media containing organic forms of nitrogen. In both peptone-meat extract and defined media with ammonium and citrate as the sole nitrogen and carbon sources, respectively, N02 , N03 , NO, and N20 were produced under aerobic growth conditions. Heterotrophic nitrification was not attributable to old or dying cell populations. Production of N02-, N03-, NO, and N20 was detectable shortly after cultures started growth and proceeded exponentially during the logarithmic growth phase. N02and N03production rates were higher for cultures inoculated in media with pH values below 7 than for those in media at alkaline pH. Neither assimilatory nor dissimilatory nitrate or nitrite reductase activities were detectable in aerobic cultures.