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Nitrous oxide (N2O) is a long-lived and potent greenhouse gas produced during microbial nitrification and denitrification. In developed countries, centralized water reclamation plants often use these processes for N removal before effluent is used for irrigation or discharged to surface water, thus making this treatment a potentially large source of N2O in urban areas. In the arid but densely populated southwestern United States, water reclamation for irrigation is an important alternative to long-distance water importation. We measured N2O concentrations and fluxes from several wastewater treatment processes in urban southern California. We found that N removal during water reclamation may lead to in situ N2O emission rates that are three or more times greater than traditional treatment processes (C oxidation only). In the water reclamation plants tested, N2O production was a greater percentage of total N removed (1.2%) than traditional treatment processes (C oxidation only) (0.4%). We also measured stable isotope ratios (delta N-15 and delta O-18) of emitted N2O and found distinct delta N-15 signatures of N2O from denitrification (0.0 +/- 4.0 parts per thousand) and nitrification reactors (-24.5 +/- 2.2 parts per thousand), respectively. These isotope data confirm that both nitrification and denitrification contribute to N2O emissions within the same treatment plant. Our estimates indicate that N2O emissions from biological N removal for water reclamation may be several orders of magnitude greater than N2O emissions from agricultural activities in highly urbanized southern California. Our results suggest that wastewater treatment that includes biological nitrogen removal can significantly increase urban N2O emissions.