Most studies examining the influence of ecological characteristics of fish on Hg concentration in fish tissues have focused on a few variables and been conducted in northern ecosystems. We examined how total length (TL), age, food-web position (estimated using delta13C and delta15N), and habitat were related to total Hg concentrations in 10 species of fish from Caddo Lake, a subtropical reservoir located on the border of Texas and Louisiana, USA. We observed biomagnification in the Caddo Lake fish assemblage, and the enrichment factors (the slope of the relationship between lamda15N and total Hg concentration) in the two habitats were 0.19 and 0.24, similar to those found in other studies. Although trophic position was the best predictor of total Hg concentration between species, age and TL were the best predictors of total Hg concentration within species. Unlike studies conducted in deep lakes, lamda13C values of fish tissue, a measure of the extent to which fish feed in food webs based on pelagic or littoral primary production, was not a good predictor of total Hg concentration in Caddo Lake fish. Total Hg concentrations in fish were elevated in forested-wetland habitats relative to open-water habitats. Data collected in the present study indicate that more Hg likely was available for incorporation into the base of the food web in the forested-wetland habitat than in the open-water habitat. Our results help to clarify the relationship between ecological characteristics of fish and Hg concentration in fish tissue and can be used by researchers as well as public and environmental health officials when designing Hg monitoring studies.