Operant discrimination learning has been extensively utilized in the study on the perceptual ability of animals and their higher order brain functions. We tested in this study whether American lobster Homarus americanus, which was previously found to possess ability of operant learning with claw gripping, could be trained to discriminate light stimuli of different intensities. For the current purpose, we newly developed a PC-controlled operant chamber that allowed the animal under a body-fixed condition to perform operant reward learning with claw gripping. Lobsters were first reinforced when they gripped the sensor bar upon presentation of a light cue. Then they were trained to grip the bar only when the light stimulus of a specific intensity was presented to obtain food reward while the stimuli of three different intensities including the reinforced one were presented in a random order. Finally, they were re-trained to grip the bar only when the light stimulus of another intensity that was not rewarded in the preceding training to obtain food while other intensities including the one that was rewarded previously were not rewarded any more. In these training procedures, the operant behavior occurred more frequently in response to the rewarded cue than to the non-rewarded one. The action latency for the reinforced stimuli showed a significant decrease in the course of training. These data demonstrate that lobsters can be trained with the light cues of different intensity as discriminative stimuli under a restrained condition that would allow application of electrophysiological techniques to the behaving subjects.