The effect of pH on surface growth and activity of Nilrobacter was studied in batch and continuous flow systems and on glass coverslips and anion-exchange resin beads. In batch culture with an initial nitrite concentration of 50 μg NO 2 −-N ml −1 freely-suspended cells had a pH optimum of 7.5 and a pH minimum of 6, which was reduced to 5.5 at lower initial nitrite concentrations. Free cells grew in continuous culture at pH 5.5 and nitrite oxidation rate per cell decreased with decreasing pH. Attachment of Nitrobacter to anion-exchange resin beads increased with pH over the range 5.5–8.0. No such increase was observed on glass coverslips but attached cells grew approximately 20% faster than free cells. Attachment to glass did not affect the pH optimum or minimum in batch culture. In continuous flow culture, the nitrite-oxidizing activity of attached cells was less than that of free cells and responses to transient changes in pH were reduced. Established attached cells were covered in extracellular slime material and significant nitrite oxidation occurred at pH 4.5. Surface growth was the major factor in the response of Nitrobacter to pH and allows for the possibility of autotrophic nitrification in acid soils.