The jaguar Panthera onca is threatened throughout its range and categorized as Near Threatened on the IUCN Red List. To inform conservation of the jaguar population in Corcovado National Park, Costa Rica, population size was estimated using data from a 3-month camera trap study. Individuals were identified from their coat patterns. The resulting density estimate of 6.98 ± SD 2.36 individuals per 100 km2 was lower than expected. The sex ratio was 1.33 males per female, and the minimum home ranges of two males were 25.64 and 6.57 km2. Hunting pressure on jaguar and white-lipped peccaries Tayassu pecari, the jaguar's main prey in the Park, may be responsible for the low jaguar density as space does not seem to be a limiting factor. The numbers of females may have been underestimated because of sampling bias and therefore the sex ratio obtained in this and similar studies must be interpreted cautiously. Better protection of the corridor that connects the Park with other protected areas is essential to guarantee long-term survival of the jaguar in Costa Rica.