Transfer and accumulation of mercury was observed to occur in a three-component estuarine food web comprising bacteria, ciliated protozoa, and copepods. Results of experiments designed to test trophic interactions revealed significant feeding of each predator on its prey. To initiate transfer and accumulation, a Vibrio sp. and Pseudomonas sp. were cultured separately in media amended with 0.2–1.0 ppm mercury, in the form of 203HgCl 2. Both bacterial species readily accumulated mercury, with the Vibrio sp. accumulating more mercury at low initial concentrations than the Pseudomonas sp., even though the Vibrio sp. was less tolerant to elevated concentrations of mercury. Washed suspensions of labelled bacteria were fed to the ciliate, Uronema nigricans, and mercury transfer was monitored by liquid scintillation counting. Phase microscopy of samples confirmed the presence of large numbers of food vacuoles within the ciliates, which was correlated with an increase in radioactivity of the ciliate fraction. The mercury concentration in protozoa was observed to be greater than that of the individual bacteria on which they fed, demonstrating magnification of mercury in the higher trophic level. The amount of mercury taken up by ciliates was dependent upon the number of mercury-laden bacteria available as food. When labelled ciliates were fed to the copepod, Eurytemora affinis, mercury concentration in the copepods was less than that of the ciliates, although greater than the initial concentration in the bacterial medium, indicating magnification above background level in the copepod. Sterile kaolinite clay to which 203HgCl 2 was added, when fed to copepods, resulted in mercury transfer and retention in the copepods, indicating that, in addition to biologically-mediated uptake, non-biological transfer to, and accumulation in, copepods also occurs.