Ephrins-A5 are expressed in the cortical target layer of thalamic afferents at the time when these axons form terminal arbors. Previous in-vitro studies provided evidence that ephrin-A5 supports the branching of thalamic axons, but there is no direct in-vivo evidence for such a growth-promoting effect. Here we examined thalamocortical projections in ephrins-A5 deficient mice. Our results demonstrate that the laminar specificity of thalamic afferents in ephrin-A5 mutants remains preserved, but axonal arbor formation is greatly reduced. Thus, ephrin-A5 specifically regulates branch formation of thalamic axons, but does not affect target layer selection. Ephrin-A5-mutant mice are, therefore, a unique model to study the effects of reduced thalamic innervation on the assembly of cortical circuits and sensory processing.