A batch nitrification process was studied using synthetic wastewater and Chilean natural zeolite as biomass carrier at ambient temperatures (20 ºC). Three groups of experiments were carried out: a first experimental set (I) with and without added zeolite using initial biomass concentrations of 1,000 and 2,000 mg VSS/L; a second set of experiments (II) with added zeolite and at the same initial biomass concentrations. In experimental set (I), only the variation of biomass concentration was measured (for both with and without zeolite), but to further test the efficiency of using natural zeolite in this wastewater treatment ammonium removal and nitrate formation over time were tested in experimental set (II). In these two experimental sets, biomass from an activated sludge process located in an urban wastewater treatment plant (WWTP) at La Farfana, Santiago de Chile, was used as inoculum (1). Finally, a third set of experiments (III) was carried out with zeolite at an initial biomass concentration of 1,000 mg VSS/L using an inoculum derived from an activated sludge process treating wastewater from a paper mill (inoculum 2). Nitrifying biomass concentration values in the range of 13,000-18,800 mg VSS/L were achieved when initial biomass concentrations varied between 1000-2000 mg VSS/L. Inoculum (1) generated higher biomass concentrations than inoculum (2). Ammonium N removals higher than 70% were obtained in experimental sets II and III. An exponential biomass growth was observed up to the second day of operation, and a slight decrease was evident afterwards, achieving stationary values after 10-12 days of operation.