A crossover tool for use with concentric tubing strings. The apparatus disclosed provides the ability to treat and/or gravel pack a producing zone in a well, and is particularly suited for use in a multiple-zone well. The crossover tool may be locked in any one of three positions: a circulate mode, a closed test mode, or a closed bypass mode to facilitate movement of the tool within the well bore. All operation of the crossover tool is effected by reciprocation of the pipe string to which it is attached. Alternative embodiments are also disclosed which dispense with the closed test mode.