In situ measurements of target strength (TS) at 38 kHz have been taken from tuna gathering around a fish aggregating device (FAD) during two cruises carried out at an interval of a year. Mean TS of -31.9 dB and -32.8 dB were measured on tuna aggregations composed mainly of 50 cm mean fork length Thunnus obesus. Mean TS were very similar between the different surveys of the same cruise. Between cruises a small difference in average TS was observed. It may be explained by the variation of the species composition of the aggregation. A greater proportion of Thunnus albacares and Katsuwonus pelamis was observed in the second year. In situ TS measurements on tuna associated with a FAD provide references of value in estimating aggregation's biomass. Furthermore TS data obtained on aggregated tuna may be useful for behavioural studies of the aggregation such as school behaviour and cohesiveness, fish length distribution, and vessel avoidance. (C) 2000 International Council for the Exploration of the Sea.