A description is given of the design construction and trials of an acoustic fish tag. The tag contains receiver and transmitter sections used in a transponding mode. The size of the unit is 5 cm long by 1 cm diameter, and it weighs 4.0 g in sea water of salinity 35%. Further development is discussed. Battery/life is dependent on the rate of interrogation. Applications include the study of fish behaviour, migration and reaction to gear, and the tracking of seabed drifters.