We report the results of an experiment designed to investigate the feasibility of using differential pressure to estimate the swimming speed and metabolic rate of Atlantic cod (Gadus morhua). Seven cod were fitted with a miniature differential pressure sensor mounted on one side of the caudal peduncle immediately anterior to the base of the caudal fin rays. Relationships between differential pressure, tailbeat frequency, tailbeat amplitude, swimming speed and rate of oxygen consumption ((O(2))) were determined as a function of the swimming speed of cod swimming at 5 degrees C in a recirculating 'Brett-style' respirometer. Tailbeat differential pressure, tailbeat amplitude and tailbeat frequency were highly correlated with swimming speed. The average or integrated pressure ranged from 0 to 150 Pa for speeds up to 0.8 m s(-1) (1.1 L s(-1), where L is total body length), while the 'pressure difference' (maximum minus minimum pressure) ranged from 0 to 900 Pa. Small changes in swimming speed of less than 0.05 m s(-1) were readily detected as differences in tailbeat pressure. Burst swimming in the respirometer resulted in huge pressure 'bursts' of up to 5000 Pa 'pressure difference'. The rate of oxygen consumption increased exponentially and was highly correlated with swimming speed (r(2)=0.77). The rate of oxygen consumption was also correlated with tailbeat integrated pressure (r(2)=0.68) and with differential pressure (r(2)=0.43); regression correlations were always greater for individuals than for combined data from all cod. The results detailed in this study indicate that an ultrasonic differential pressure transmitter would enable accurate estimates of the swimming speed, rates of oxygen consumption and activity patterns of free-ranging fish in nature.