An optical switching OEIC (optoelectronic integrated circuit), consisting of a GaAs MESFET and an InP/InGaAsP 2 x 2 buried waveguide optical switch, is reported for the first time. Fully monolithic integration is obtained by heteroepitaxial GaAs-on-InP growth and photolithographical interconnection. The combined OEIC allows switching voltages of about 1 V with an extinction ratio of -6 dB.