Previous studies showed the Fmr1 knockout (KO) mouse to be an excellent animal model for human fragile-X syndrome. The aim of this study was to further characterize the phenotype of these animals. Neuroanatomically, KO male mice were compared to wild-types (littermates) with respect to their sizes of hippocampal intra- and infrapyramidal mossy fiber (IIPMF) terminal fields. Behaviorally, they were tested in four different paradigms, each measuring different aspects of cognitive and emotional behavior: elevated plus maze (anxiety), neutral cage (aggression), open field (exploration), and radial maze (spatial memory). The results showed a diminished ability for radial maze learning associated with smaller sizes of IIPMF terminal fields. In addition, Fmr1 knockout animals exhibited increased locomotor activity, while no differences were found for aggression and anxiety. These data suggest the involvement of FMRP protein in the development of spatial learning and the sprouting of IIPMF terminal fields.