Media coverage has made the public aware of both the benefits and the risks from eating self-caught fish, but information on contaminants in commercial fish is much more limited, especially on a local level. The U.S. Food and Drug Administration website provides methylmercury data for a variety of fish, but for many species sample sizes are small and data are more than a decade old, whereas commercial fish sources are highly dynamic. A few state agencies are beginning to provide contaminant information for commercial fish, including canned tuna. We examined the mercury concentration of six types of fish purchased in supermarkets in Chicago, Illinois in 2005. We measured total mercury (methylmercury accounts for about 90% of the total mercury in fish). One key question was whether the concentrations of mercury in fish available locally were similar to those reported in other areas of the country and in the FDA U.S. national data base. Such information is critical for the public, especially pregnant women or those planning on pregnancy, making decisions about types and quantities of fish to consume. Some fish are available generally throughout the U.S., but others are more locally available, suggesting a need for site-specific information. This research was stakeholder driven, and reflected local interest in the safety of local fish. There were significant differences in mercury concentrations among the fish, ranging from a mean of 0.03 microg/g (ppm-wet weight) for salmon (Salmo spp.) to 1.41 ppm for swordfish (Xiphias gladius). Maximum values for three species of fish (orange roughy Hoplostethus atianticus, swordfish walleye Sander vitreus) were over 1 ppm (FDA action level), and all of the fish except salmon had some values above 0.5 ppm, the action level set by some states and countries. There were significant differences in mercury concentrations among three types of canned tuna (Thunnus spp): "gourmet tuna" had the least amount of mercury, and white tuna had the most. The mean concentrations reported in this study were generally similar to those reported by the FDA, but there were important differences: 1) although the mean mercury concentrations for orange roughy for the Chicago data was similar to the FDA data, the maximum concentration was higher; 2) the mean for the Chicago swordfish was higher than the FDA data (1.26 vs 0.97 ppm, methylmercury); 3) the maximum for tuna steaks was higher in the FDA data set; and, 4) mean values for grouper (Epinephelus spp.) were higher in the FDA data set than the Chicago data. Further, the FDA has virtually no data on walleye and none on "gourmet tuna". These conclusions suggest that there are enough variations between the local data (Chicago) and the FDA data to warrant periodic local monitoring of commercial fish to provide up-to-date information to consumers about mercury in the fish they eat.