A tool for use in initiating a hydraulic fracture in a bore hole comprises an elongate cylindrical bore hole packer structure (11) having an inner longitudinal passage (12), a mid-portion (13) provided with ports (14) extending outwardly from passage (12) to the exterior periphery of the packer structure (11) and expandable circumferential well portions (15A) surrounding the inner longitudinal passage (12) to each side of the mid-portion (13). In use of the tool the circumferential wall portions (15A) can be expanded by injection of hydraulic fracturing fluid into passage 12) and exit of the injected fluid through the ports (14) to produce a pressure difference between the inside of the packer structure and the outside of the packer structure as the fluid passes through the ports (14) such that the hydraulic fluid exiting the packer structure can initiate a fracture. The packer structure (11) is disposed between a pair of tool end pieces (16) one of which provides a fluid inlet for injection of hydraulic fluid into one end of passage (12) and the other of which closes the other end of passage (12) against outflow of hydraulic fluid therefrom.