Individuals affected by Fragile X syndrome (FXS) experience cognitive impairment, hyperactivity, attention deficits, social anxiety and autistic-like behaviors. FXS results from the loss of expression of the Fragile X mental retardation (FMR1) gene, whose protein product FMRP is thought to play an important role in neuronal function and synaptic plasticity. Two paralogs of FMRP, FXR1P and FXR2P, have been identified, forming the Fragile X-related (FXR) family of proteins. Although the functions of FXR1P and FXR2P are not well understood, there are similarities among all three FXR proteins in gene structure, amino acid sequence, expression pattern and cellular functions. Mouse models have been described for loss of Fmrp, Fxr1p and Fxr2p, the mouse homologs of FMRP, FXR1P and FXR2P. In earlier studies, we found that Fmr1 knockout (KO) mice, which do not express Fmrp, and Fxr2 KO mice, which do not express Fxr2p, show similarities in some behavioral responses such as hyperactivity. To better understand the functional relationship between FMRP and FXR2P, we generated Fmr1 KO, Fxr2 KO, Fmr1/Fxr2 double KO and wild-type control mice as littermates on the same genetic background and examined them in several behavioral assays. Results show that Fmr1/Fxr2 double KO mice have exaggerated behavioral phenotypes in open-field activity, prepulse inhibition of acoustic startle response and contextual fear conditioning when compared with Fmr1 KO mice, Fxr2 KO mice or wild-type littermates. Our findings suggest that Fmr1 and Fxr2 genes contribute in a cooperative manner to pathways controlling locomotor activity, sensorimotor gating and cognitive processes.