Using a simultaneous discrimination procedure it was shown that pigeons were capable of learning to discriminate 100 different black and white visual patterns from a further 625 similar stimuli, where responses to the former were rewarded and responses to the latter were not rewarded. Tests in which novel stimuli replaced either the rewarded or nonrewarded stimuli showed that the pigeons had not only learned about the 100 positive stimuli but also about the 625 negative stimuli. The fact that novel stimuli enhanced discrimination performance when they replaced the many negative stimuli indicated that the pigeons had categorized the stimuli into two classes, familiar and less familiar. Long-term retention was examined after a 6-month interval. To begin with it seemed poor but a recognition test performed after the subjects had been retrained with a subset of the stimuli after an interval of 7 months confirmed that pigeons are capable of retaining in memory several 100 visual items over an extended period. It is proposed that the initial retrieval weakness was due to a forgetting of the contingencies between stimulus categories and response outcomes. Further tests involving variously modified stimuli indicated that while stimulus size variations had a negative effect on performance, orientation changes did not interfere with recognition, supporting the view that small visual stimuli are memorized by pigeons largely free of orientation labels. The experiment generally confirms that pigeons have the capacity of storing information about a large number of visual stimuli over long periods of time.