The aim of the study was to evaluate the removal of ammonium ions from wastewaters using a naturally occurring zeolite (clinoptilolite) from Rupea (Brasov country, Romania). Experiments to determine the effect of the zeolite dosage (1 to 10 g/100 ml wastewater), grain size (0.063 - 1.0 mm and 1.0 - 3.0 mm) and contact time (5 to 90 minutes) on the NH4+ removal efficiency were performed using the bach technique. The concentration of the ammonium ions in the wastewater before and after the contact with Rupea zeolite was determined by UV-VIS spectrophotometry. The obtained results indicate that the investigated zeolite was effective in the removal process of the ammonium ions from water. The concentration of the NH4+ decreases below the maximum consent limit, after 15 minutes of contact with 2 g zeolite (0.063-1 mm).