Effects of mains frequency (50Hz) magnetic fields (MF) on behaviour of captive Atlantic salmon were examined in a large arena. Fish swam spontaneously within the arena passing from one side to the other through Helmholtz coils (arranged in four pairs) that penetrated a net barrier. When activated, a uniform intensity MF was generated between coils within a pair. Salmon were considered in two size categories: large fish (62-85 cm) and smaller post-smolts (24-41 cm). Large fish were tested in groups of six and considered at the individual level. For each fish, measurements were made of the times to approach, traverse and leave an activated coil pair (intensity = 95 µT) on first encounter across defined distances. These times were compared with previous passage times for the same coil in a deactivated state across similar distances. There were no significant differences in approach, traverse or departure times associated with activation state of the coils. Post-smolts (18 trials of groups of six fish) were exposed to three 30-min periods of MF at 1.3, 11.4 and 95 µT with 30-min controls before each treatment. The sequence in which the different MF levels were presented was varied sequentially. There was no evidence that the numbers of fish passing through the coils depended on the sequence in which 2 the MF intensities were presented (p = 0.18) or on the field intensity itself. Similarly, the numbers of shoals passing through coils did not depend on sequence or field intensity. There were no observations of unusual behaviours in association with MF up to 95 µT.