Abstract: This is a subsurface safety valve for insertion in a tubing string centered in a well drilled in the earth normally for the production of oil or gas. Under normal flow conditions, a spring holds a flapper valve in an open position out of the flow path of the fluid. An increase in the temperature of the fluids flowing through the valve supplies the force to push the flapper into the flow stream where the fluid flow will cause it to close. A heat-sensitive fluid is contained in an annular reservoir surrounding the flow path of the fluid through the valve and an annular piston is in one end of the cylinder. Expansion of the fluid causes movement of the piston which has a rod which forces the flapper valve into the main fluid flow stream.