Operation of the first complementary GaAs MESFET (CMES) logic gates is reported. Direct-coupled inverters utilizing p- and n-channel ion-implanted MESFET's demonstrate good transfer characteristics with less than 5-µW power dissipation per gate. Propagation delays as small as 54 ps are attained in 13-stage ring oscillators at room temperature with speed-power products as small as 6 fJ.