This paper proposes target-strength (TS) to fork length (FL) and swimbladder volume relationships, respectively, for bigeye (Thunnus obesus) and yellowfin (T. albacares) tuna, based on previous TS measurements. TS versus FL relationships were determined using the equations TS=a logFL+b but the results should be regarded as preliminary because of the small number of observations. The TS of bigeye tuna was about 5 dB higher than yellowfin when comparing species at equal size. This result can be linked to the swimbladder volume difference between species. For both species a single trend is observed in the relationship between TS and swimbladder volume. This result confirms that this is the key factor in deciding the TS of physoclists fish. Since TS measurements were obtained from as deep as 500 m it would seem thai a tuna's swimbladder is not fully compressed and maintains its cross-section even at great depths. Therefore tuna have the ability to control efficiently the volume of their swimbladders. (C) 2000 International Council for the Exploration of the Sea.