Variation in avian bilateral symmetry can be an indicator of development instability in response to a variety of stressors, including environmental contaminants. We used composite measures of fluctuating asymmetry to examine the influence of mercury concentrations in two tissues on fluctuating asymmetry within four waterbird species. Fluctuating asymmetry increased with mercury concentrations in whole blood and breast feathers of Forster's terns (Sterna forsteri), a species with elevated mercury concentrations. Specifically, fluctuating asymmetry in rectrix feather number one was the most strongly correlated structural variable of those tested (wing chord, tarsus, primary feather number 10, rectrix feather number 6) with mercury concentrations in Forster's terns. However, for American avocets (Recurvirostra americana), black-necked stilts (Himantopus mexicanus), and Caspian terns (Hydroprogne caspia), we found no relationship between fluctuating asymmetry and either whole blood or breast feather mercury concentrations, even though these species had moderate to elevated mercury exposure. Our results indicate that mercury contamination may act as an environmental stressor during development and feather growth, and contribute to fluctuating asymmetry of some species of highly contaminated waterbirds. This article is protected by copyright. All rights reserved.