Neurexin 1? mutations are strongly associated with neurodevelopmental disorders such as autism spectrum disorders and schizophrenia in humans. Studies using the Neurexin 1? knock-out mouse have showed behavioral abnormalities of relevance to these disorders and baseline deficits in excitatory synaptic function have been described. However, little is known about the effect of Neurexin 1? deletion on behavior during development. This study examined the effects of Neurexin 1? deletion on behavior across a range of developmental time points to determine whether potential abnormalities follow a developmental trajectory. Pups lacking Neurexin 1? emitted a reduced number of ultrasonic vocalizations early in development combined with a restricted repertoire of calls indicative of a loss in complexity in vocal production and showed delays in reaching certain developmental milestones. Behavioral testing showed that juvenile and adult male Neurexin 1? knock-out mice exhibited social deficits and increased levels of aggression, confirming previous findings. No increases in repetitive behaviors or deficits in motor learning or olfaction were seen. In conclusion, this research showed that Neurexin 1? deletion does result in social and communication deficits that follow a developmental trajectory. These are the first experimental data that associate a deletion of Neurexin 1? with alterations in behaviors relevant to autism spectrum disorder across development and highlight the importance of assessing the developmental trajectory in mouse models of neurodevelopmental disorders.