This method is for determination of methyl mercury (CH3Hg) in filtered and unfiltered water by distillation, aqueous ethylation, purge and trap, desorption, and cold-vapor atomic fluorescence spectrometry (CVAFS). This method is for use in EPA's data gathering and monitoring programs associated with the Clean Water Act, the Resource Conservation and Recovery Act, the Comprehensive Environmental Response, Compensation and Liability Act, and the Safe Drinking Water Act. The method is based on a contractor-developed method (Reference 1) and on peer-reviewed, published procedures for the determination of CH3Hg in aqueous samples, ranging from sea water to sewage effluent (References 2-7).