This study focused on the cytoarchitectonic and morphological differences in GABA-releasing interneurons between adult Fmr1 knock-out (FMR1KO) and wild-type (WT) mice in the somatosensory cortex. Our results showed a robust reorganization of neocortical, but not hippocampal inhibitory circuits in the FMR1KO mouse. The reorganization is characterized by a significant reduction (20%, p<0.001) in the densities of parvalbumin (PV)-positive, but not calbindin (CB) and calretinin (CR)-positive interneurons. A significant enlargement of soma size and an altered lamina distribution of PV but not CR and CB cells was also observed. Additionally, there was a modest but significant increase in TrkB-immunoreactivity in PV-positive cells in the FMR1KO mouse. These results provide the first report showing significant alterations of GABA-releasing interneurons in the mouse model of fragile X syndrome. Uncovering the changes in specific GABAergic inhibitory circuits could help understand mechanisms underlying the behavior deficits of fragile X syndrome and autism.