Seasonal distribution, size classes, density, biomass, population structure, and sex ratio of Trachemys dorbigni were investigated in Delta do Jacui State Park, Rio Grande do Sul — Brazil, between 2003 and 2006 using mark-recapture methods. A total of 137 Trachemys dorbigni individuals were captured: 93 females, 30 males, and 14 undetermined. Recapture rate was 5.1%, with a total of 7 recaptures (one animal was recaptured twice and 5 other animals were recaptured once). Individuals in these populations were active between August and April, and no turtles were captured in February. Sexual dimorphism index (i.e., females' carapace length divided by males' carapace length) was 1.13, and females were bigger than males in all analyzed variables, except tail base to the cloaca opening length. Density was 7.61 turtles/ha, and biomass was 8.71 kg/ha. The distribution of body size classes revealed that the samples were composed mainly of adults (89.78%). Sex ratio was different from 1:1 (3:1; χ² = 32,27; df = 1; p < 0.001).