Abstract: A test tool is provided for testing the production capabilities of a preselected formation subsequent to the drilling of a subterranean well. The test tool is part of a testing string which incorporates a packer which may be releasably engaged in the casing string at a depth immediately above a region where formation testing is desired. The test tool includes a rotary ball valve which is normally maintained in a closed position during the insertion of the tool into the well. After setting of the packer, the ball valve is opened by increasing the fluid pressure existing in the annulus between the casing and the testing string. Such annulus pressure is applied to a first reservoir of trapped fluid which may contain water, and supplies fluid at annulus pressure to one side of a valve operating piston to shift the operating piston to actuate the rotary ball valve to its open position and compress a piston return spring.