Neuronal network formation in the developing nervous system is dependent on the accurate navigation of nerve cell axons and dendrites, which is controlled by attractive and repulsive guidance cues. Ephrins and their cognate Eph receptors mediate many repulsive axonal guidance decisions by intercellular interactions resulting in growth cone collapse and axon retraction of the Eph-presenting neuron. We show that the Rac-specific GTPase-activating protein a2-chimaerin binds activated EphA4 and mediates EphA4-triggered axonal growth cone collapse. a-Chimaerin mutant mice display a phenotype similar to that of EphA4 mutant mice, including aberrant midline axon guidance and defective spinal cord central pattern generator activity. Our results reveal an achimaerin-dependent signaling pathway downstream of EphA4, which is essential for axon guidance decisions and neuronal circuit formation in vivo.