The objective of this research was to include ozonation prior to an activated sludge treatment and investigate the effect on the nitrogen species, their fate and the consequences of this oxidation upon the biomass. Three parallel treatment systems were used: the base system, where feed went directly to the activated sludge reactor, and two others, where the influent was ozonated at two different dosages, 15 and 25 mg/L of influent, prior to the biological reactors. The results from the ozonation chamber show a high oxidation capacity of the entering ammonia and organic nitrogen, proportional to the ozone dose. The oxidation product was nitrate. No de-nitrification was expected because a high oxygen concentration (4 mg/L) was maintained in the reactors. The reactors receiving ozonated influent showed a lower assimilation of nitrogen by the biomass. The sludge nitrogen content resulted in 11, 9.3 and 7.4% dry-weight corresponding to no-ozone, low ozone and high ozone dosages, respectively. In spite of the lower ammonia available in the ozonated flows, the corresponding reactors showed a higher specific nitrification rate. The ozonated system also performed better in terms of chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) removals, besides showing a higher true biomass yield coefficient.