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The purpose of this experiment was to investigate the generalization and long-term retention of equivalence relations in individuals with mental retardation. To date, the generalization of equivalence relations to a range of novel stimuli has only been demonstrated among verbally competent adults. The responding of many individuals with mental retardation often fails to come under control of relevant stimulus features and fails to generalize to novel stimuli. Thus, we assessed whether the generalization of stimulus equivalence would occur in the absence of remedial training for 4 adults with mild or moderate mental retardation. Subjects learned 6 conditional discriminations and were tested for the emergence and generalization of 3 symmetry and 3 equivalence relations. Subjects were also tested for their retention of the relations approximately 1 -4 months following their last laboratory session. All subjects showed the emergence and generalization of all of the relations in the absence of remedial training, and most showed the long-term retention of the relations. Accuracy improved during the retention test as a function of repeated testing for some subjects.