Despite the high species richness of the family Chelidae (side-necked turtles) in South America, few demographic studies are available for a neotropical chelid turtle. Here we report data from a long-term (1993–2006) study of Hydromedusa maximiliani. Population size was estimated for two periods (1993–1994 and 2003–2006) by the Jolly-Seber capture-mark-recapture method and the demographic parameters were estimated by Cormack-Jolly-Seber model. The population size estimates showed values that increased over the study period (from 235 to 318 individuals in a 250 ha area), although this result must be viewed with caution despite the large confidence intervals of the estimates. Significantly fewer individuals of intermediate body sizes (80–100 mm plastron length) were present in the 1993–1994 period compared to the 2003–2006 period. The population was female biased with male:female ratio of 1∶2. Survival rate (ϕ) was considerably higher for adult females than for males. The estimated annual population growth rate (λ) was 1.012, and the estimated temporal process variance, the variation in the population growth rate, was practically null. These demographic parameters indicate a healthy population, probably reflecting the protected status of the study area.