We estimated demographic parameters for Acanthochelys spixii in Parque Nacional de Brasí lia, Distrito Federal, Brazil, based on 4 years of data collection, and also analyzed the effects of sex, temporal variation, and climatic factors on population dynamics. The adult sex ratio did not vary significantly from 1∶1. By using the Akaike's information criterion, selection of candidate models constrained for climatic variables indicated that the interaction between total rainfall and average air temperature from the previous month as well as recapture probability (p) on a monthly basis explained most variation in demographic parameters, with a constant annual apparent survival (Φ) value of 0.82. Recapture rates during the study period varied from 0.01 to 0.23 and, during the dry season, from 0.01 to 0.04. The monthly average population size was 30 adult turtles, with values between 10 and 35 adults over 4 years. The annual population growth rates were 1.37 for 2005–2006 and 0.59 for 2006–2007. The small population size of Acanthochelys spixii seems characteristic of chelids in the area, and maintaining it depends on preserving natural ecosystems inside Parque Nacional de Brasília.