An electro‐optic directional coupler switch has been fabricated with a GaAs homojunction structure and tested at 1.15 &mgr;m. Each single mode guide is made in an‐LPE layer grown on an+substrate, thep+rib used to confine the light being obtained by Zn diffusion and chemical etching. A calculation of the losses in the planar approximation roughly yields the large values of attenuation measured for TE and TM modes (∼8 cm−1). By reversely biasing the ’’stepped &Dgr;&bgr;’’ junctions with less than 17 V, more than 13 dB power isolation has been achieved for both switching states on a 6.3‐mm‐long device. Large improvements of the characteristics are expected from the additional design parameters provided by GaAs‐GaAlAs double heterostructure.