The fragile X [fra(X)] syndrome is manifested phenotypically as a developmental disability comprised mainly of moderate-to-severe mental retardation (MR). Deficits are especially evident in auditory and visual short-term memory. Recently, an FMR1 knockout mouse developed by the Dutch-Belgian Fragile X Consortium demonstrated significantly lower visual-spatial abilities than littermate controls. We wondered if these results were associated with learning per se or to performance deficits only. Thus, we examined learning and memory in male FMR1 knockout mice crossbred from Fvb and E129 strains, and in male Fvb control mice, using operant conditioning techniques. In Experiment 1, we demonstrated that two aged male FMR1 knockouts could acquire the necessary bar-press response to discriminate visual (L+) and auditory (N+) stimuli. In Experiment 2, we showed that three naive male knockouts and two naive male controls, all 12 weeks old, also learned to discriminate L+ and N+. A third component, a complex discrimination task, during which light and noise were presented concurrently without reinforcement (LN-) was added to each session. All knockouts acquired both L+ and N+ discriminative responses in fewer sessions and with higher discrimination ratios than either control. Moreover, all knockouts exhibited the typical response pattern associated with complex discrimination (LN-) tasks. However, neither control made the complex discrimination. Our findings were unexpected and raise issues concerning FMR1 mouse strains and their cognitive-behavioral testing.