Formate and acetate as well as other organic and inorganic anions have been measured in precipitation collected in Colorado at Niwot Ridge (a rural site) and Boulder (a suburban site). At both sites, the organic anion concentration is usually at least 20% of the nitrate concentration and occasionally exceeds the nitrate. Formate is the dominant organic anion measured with concentrations as large as 9¿10-5 M occurring in summer rain showers. A variety of dicarboxylic anions are observed but their concentration is generally much less than formate. Correlation with nitrate suggests that the large values found for the organic anions results from anthropogenic pollution.