Fragile X syndrome (FXS) is the most common inherited form of intellectual disability, and is caused by mutations in the FMR1 gene that lead to loss of the protein it encodes, FMRP. Many fragile X-related cognitive and behavioral features emerge during childhood and are associated with abnormal synaptic and cellular organization of the cerebral cortex. Yet, how the loss of FMRP influences the developmental trajectory of cortical maturation remains unclear. To monitor cortical maturation, we exploited the stereotyped development of the organization of anatomical structures in layer 4 of the murine primary somatosensory cortex (S1), called barrels, which recapitulate the pattern of whiskers on the snout. Immunohistochemistry revealed FMRP throughout S1 in wild-type (wt) mice (n=3-4) during postnatal development, with its highest expression corresponding to times critical to barrel formation and synaptogenesis. To determine the effect of the loss of FMRP on S1 development, we compared cortical maturation in Fmr1 knockout (Fmr1-/y) and wt mice. Whereas cortical patterning and dendrite complexity in layer 4 are not altered in Fmr1-/y mice compared to wt, loss of FMRP leads to a delay in the emergence of fine features of barrel cytoarchitecture and a decrease in the synaptic levels of proteins involved in glutamate receptor signaling at times corresponding to the highest levels of FMRP expression. Quantification of barrel segregation expressed as the ratio of the density of layer 4 cells in barrel walls to barrel hollows revealed a reduction in Fmr1-/y mice compared to wt at postnatal day 7 (P7) [wt: 1.56 ±0.04 (SEM), n=14; Fmr1-/y: 1.42 ±0.04, n=14; p=0.03 by unpaired Students t test], but not at P14. To look at the role of FMRP in dendritic spine shape, we classified spine morphology on individual fluorescent dye-filled S1 layer 4 spiny stellate neurons in Fmr1-/y and wt mice at P14. Loss of FMRP led to an immature dendritic spine shape profile, with a greater proportion of filopodia [wt: 16.58 ±2.70 (SEM), n=6; Fmr1-/y: 26.56 ±2.25, n=4; p=0.042 by MANOVA], and a decrease in the proportion of mushroom shaped dendritic spines [wt: 54.30 ±3.77 (SEM), n=6; Fmr1-/y: 42.71 ±2.34, n=4; p=0.042 by MANOVA] in Fmr1-/y mice compared to wt. Consistent with this, Western blotting of homogenates and synaptoneurosomes from P7 and P14 Fmr1-/y and wt neocortex (n=3-6) indicated that loss of FMRP causes a transient decrease in synaptic levels of proteins involved in glutamate receptor signaling. The specificity of the developmental delay in Fmr1-/y mice indicates that loss of FMRP does not result in a general stalling of cortical maturation. Instead, our results suggest that inaccurate timing of developmental processes caused by the loss of FMRP may lead to changes in neural circuitry that underlie behavioral and cognitive dysfunctions associated with FXS.