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This paper presents the results of a survey of N-nitrosodimethylamine (NDMA) occurrence in treated waters of 21 North American drinking water treatment plants. Quarterly samples of treatment plant influent, finished, and distribution system waters were collected and analyzed for NDMA as part of an American Water Works Association Research Foundation and Water Environment Research Foundation jointly funded study. Sampled systems were located in seven states of the United States and four provinces of Canada and comprised systems with pristine sources as well as systems downstream of wastewater treatment plants. Various treatment processes and disinfectants were represented as were various detention times and pipe materials in the distribution system. Four quarters of samples were analyzed for most of the systems. Information on source water characterization, treatment processes, and treated water distribution system parameters was also collected for many systems. Most NDMA analyses were performed by solid-phase extraction with a carbon resin (Ambersorb 572; SUPELCO, Bellefonte, Pa.) and gas chromatography (GC)/high-resolution mass spectrometry (HRMS). The samples all had NDMA values below 7 ng/L except for two systems, which had NDMA values above 10 ng/L. An NDMA level above the method reporting limit (MRL), from 0.6 to 1 ng/L, was detected in only one of 81 plant influent water samples. Values for NDMA in 81 plant effluent samples ranged from below the MRL to 30 ng/L and in 95 treated water distribution samples from below the MRL to 24 ng/L. Matrix spike recoveries ranged from 80 to 120 percent, with an average of 94 percent. In addition, 12 samples were analyzed by a reference laboratory, with comparable results. The relative differences in replicate analyses ranged from zero to 16 percent, with an average difference of 6 percent. The relationship of NDMA levels with other parameters (e.g., source water, treatment processes, disinfectants, detention time) is being evaluated to gain insight on formation and degradation of NDMA. Includes 6 references, tables, figures.