We conducted a mark-recapture study on snapping turtles (Chelydra serpentina) at the headwaters of the San Marcos River, Hays County, Texas. The site is within a highly modified urban environment partially surrounded by a golf course, athletic fields, remnants of a theme park, and roadways supporting high traffic volume. We conducted the study from 1996–2011. We captured 179 turtles (89 adult females, 77 adult males, and 13 juveniles). We recaptured males significantly more frequently than females. We estimated population size to be 215 individuals with a density of 26/ha. The annual estimated probability of capture was 0.33, and estimated annual survivorship was 0.94 for males, 0.93 for females, and 0.81 for juveniles.