Bilateral integration of sensory and associative brain processing is achieved by precise connections between homologous regions in the two hemispheres via the corpus callosum. These connections form postnatally, and unilateral deprivation of sensory or spontaneous cortical activity during a critical period severely disrupts callosal wiring. However, little is known about how this early activity affects precise circuit formation. Here, using in utero electroporation of reporter genes, optogenetic constructs, and direct disruption of activity in callosal neurons combined with whisker ablations, we show that balanced interhemispheric activity, and not simply intact cortical activity in either hemisphere, is required for functional callosal targeting. Moreover, bilateral ablation of whiskers in symmetric or asymmetric configurations shows that spatially symmetric interhemispheric activity is required for appropriate callosal targeting. Our findings reveal a principle governing axon targeting, where spatially balanced activity between regions is required to establish their appropriate connectivity. •Callosal wiring defects result when early cortical activity is bilaterally dissimilar•Functional callosal circuits form after bilateral, but not unilateral, whisker removal•Normal callosal wiring requires balanced sensory or endogenous cortical activity•Bilateral spatial symmetry of whisker input is instructive for callosal wiring The corpus callosum connects neurons between the brain hemispheres, and its development is sensitive to neuronal activity. Suárez et al. find that a spatially symmetric interhemispheric balance of cortical activity is required during development for correct targeting of callosal axons.