A factor that controls maximum cell-specific nitrite oxidation rate was examined using the enrichment culture of Nitrobacter. Increasing or decreasing nitrite loading changed the content of Q-10, which served as a hydrogen carrier in the electron transport chain of Nitrobacter, which varied the maximum cell-specific nitrite oxidation rate. That is, the amount of oxidized nitrite, which is the sole electron donor, changed in the maximum cell-specific nitrite oxidation rate. Then, a dynamic change in maximum cell-specific nitrite oxidation rate was examined at short time intervals with increasing nitrite loading. Consequently, it was found that the maximum cell-specific nitrite oxidation rate began to increase after reaching a certain amount of oxidized nitrite. It was additionally indicated that the maximum cell-specific nitrite oxidation rate presumably had an upper limit and increased gradually near the limit. From the results, the response time and variation in maximum cell-specific nitrite oxidation rate are likely to be determined on the basis of the hysteresis of nitrite oxidation by nitrite-oxidizing bacteria.