Podocnemis expansa (the giant South American or arrau turtle) is a freshwater turtle in critical risk of extinction, distributed along the Orinoco, Amazon, and Esequibo river basins. Protection of nesting beaches and headstarting of hatchlings has been in progress since 1989 on the Arrau Turtle Wildlife Refuge located in the Middle Orinoco River in Venezuela. The purpose of our study was to evaluate the population dynamics of the species in this refuge. We used data on the number of turtles and number of nests collected from 1989 to 2004 that were available from the Venezuelan Ministry of Environment as well as data collected by us in 2004. Also, we collected data in 2004 on sizes of individuals of known age. First, a somatic growth curve was constructed and used to transform size structure in age structure. Age-specific fecundity and survivorship were calculated. Finally, we constructed a size-dependent matrix model and used it to determine population status and to assess the effectiveness of alternative management options. According to our analyses, the population size and age structure have stabilized. The analyses also indicate that protecting the survivorship of juveniles and adults, more than fecundity, is the conservation action that could best reverse a population decline. We conclude that additional efforts focused on protecting juveniles and adults of the arrau turtle, such as reinforcement of illegal hunting controls, must be implemented in order to ensure the success of the conservation program.