The Amazon and its tributaries are home to almost one tenth of the world’s freshwater fish biodiversity, 2500 species being recorded. The giant fish Arapaima gigas is one of the most emblematic of these. However, this graceful predator is a victim of overfishing, and is now on the IUCN Red List of Threatened Species. Catches in the wild have been declining substantially and fish farming is gradually being developed in the Brazilian and Colombian Amazon to alleviate this. Captive reproduction is one of the main problems hampering aquaculture of Arapaima notably because it is almost impossible to distinguish the sex of the reproductive adults. A new, practically non-invasive, method based on testing of a straightforward blood sample, enabled an IRD team and its Peruvian partner to determine accurately the sex of 100% of nearly 30 specimens. The technique should help improve breeding programmes of this species and at the same time facilitate study and monitoring of wild populations to improve conservation and management strategies.