A research programme was carried out in French Polynesia to study tuna behaviour using acoustics and fishing experiments. Acoustics are of great importance for the study of tuna behaviour and estimation of abundance but estimates of individual target strength that are available are particularly inaccurate. In this study, four yellowfin tuna (Thunnus albacares) and two bigeye tuna (Thunnus obesus) of weight 4-50 kg were individually caught, identified, and equipped with ultrasonic tags for telemetry experiments. While tracking the fish, simultaneous underwater acoustic data were recorded with a split-beam echosounder in order to estimate their in situ acoustic target strength. It was observed that target strength was stronger when fish were diving than when they were ascending toward the surface. This can be explained by the tilt angle orientation of the swimbladder. A target strength bias according to depth was also observed.