The invention is directed to a gate valve assembly which comprises an elongated body having a longitudinal bore extending therethrough; a plurality of gates slidably disposed in the longitudinal bore across respective flow passages which each extend laterally through the body between corresponding inlet and outlet ports; each gate being movable between an open position wherein an opening in the gate is aligned with its respective flow passage and a closed position wherein the opening is offset from its respective flow passage; and an actuator for moving the gates simultaneously between a first position, wherein at least one of the gates is in its open position, and a second position, wherein the at least one gate is in its closed position.