Population monitoring and evaluation of spatial requirements of species are key actions for the conservation of wild populations, especially for endemic and threatened species. We estimated the abundance of the endemic species Mesoclemmys dahli in 2 streams in Cesar, Colombia, from February 2008 to February 2009, by using aquatic sampling. We monitored fluctuations in abundance through the year and used mark–recapture sampling and the Jolly–Seber model to estimate population size and density. Home ranges and movement patterns were studied by using very high frequency (VHF) radio telemetry. We calculated home ranges for the year and for the wet and dry season independently. Estimated population size for the 2 streams varied during the year from 16 (95% CI, 7–30) to 175 (95% CI, 32–298) turtles. Densities ranged from 16 turtles/ha in April to approximately 170 turtles/ha in June, which is lower than other populations of M. dahli in Colombia as well as other South American chelids. Individuals captured during the wet season had a significantly higher mean body mass than those captured during the dry season, which suggests that larger animals may leave the streams during the dry months or that animals may lose weight during this period. Home ranges for 1 year varied from 1.6 to 30.8 ha when using the minimum convex polygon method and from 9.2 to 22.5 ha when using the fixed kernel density estimator. There were no significant differences in the mean movements during the dry and wet seasons. However, the greatest movements were documented either during the transition from wet to dry months or during the dry months. Conservation of M. dahli will require protection of the few streams where it occurs, as well as the associated riparian vegetation and surrounding areas used by the species.