Abstract: An improved shuttle valve for providing control of fluid from two alternate ources to a fluid-actuated mechanism. Primary and secondary fluid sources are coupled to supply fluid to a shuttle valve for transmission to a single outlet port. A bias piston having a passage axially extending therethrough is slidably positioned to engage the shuttle and impart movement in response to fluid pressure from the primary source. A bias source is coupled to vary the pressure on the piston and thus change the net bias needed to allow fluid from the primary source to maintain the shuttle in a first position. Initially, when the shuttle is in the first position, fluid flows from the primary source to the single outlet. Subsequently, when the pressure from the primary source falls below a predetermined value, the fluid pressure from the secondary source moves the shuttle to enable fluid flow from the secondary source to the single outlet.