Although conditioning techniques are the most powerful way to study behavioural responses by animals to external stimuli, the magnetic sense has proved surprisingly resistant to conditioning approaches. This study demonstrated learned discrimination of magnetic field intensity stimuli by a new species, the rainbow trout (Oncorhynchus mykiss). In a unitary conditioned discrimination technique, four juvenile rainbow trout were trained to strike a target at the end of a response bar in anticipation of food. In successive experiments, the trout failed to discriminate the presence and absence of a vibration stimulus, but subsequently learned to discriminate the presence and absence of a magnetic field intensity anomaly (peak intensity of 75 μTesla). The authors conclude that the necessary conditions for training animals to magnetic intensity are the use of spatially distinctive stimuli and of a conditioned response that requires movement.