Global location sensors (GLS) are increasingly being used to determine animal position at sea. Their small size and weight means that they can be attached to the leg of volant birds with supposedly little impact on the flight ability. However, very few studies have investigated the impact that foot-attached devices may have on the diving ability of foot-propelled seabirds. We compared the diving activity of two groups of free-ranging great cormorants Phalacrocorax carbo carbo, both groups carrying identical time-depth recorders attached to the tail, and one group also having leg-attached GLS. Our results showed that there were no differences between the two groups in any of the diving parameters investigated, at least over the short term. Caution should be exercised when extrapolating to other species, especially those smaller than great cormorants, and also when deploying GLS over longer periods.