It is postulated that nitrification in fixed films can be achieved only when nitrifiers embedded in the surface of a growing biofilm of mineralizing bacteria get sufficient time in the aerobic zone of the biofilm to multiply and reach such concentrations that ammonia conversion becomes significant. This phenomenon is modelled analytically under simplified assumptions. The developed criterion is compared with data from the literature and found qualitatively adequate -though the investigations often lack data on parameters, like the oxygen concentration, which theoretically has been found to be the rate limiting substrate.