A powder fire extinguisher contains powder which will be expelled, through an outlet passage (16) by a compressed gas. The gas is contained in a cylinder which is punctured to operate the extinguisher. A valve (31,32) is positioned in the outlet passage, and when the gas cylinder is punctured, the gas pressure immediately acts on the valve (31,32) to hold it closed until pressure has built up in the main body of the extinguisher and the powder is completely fluidized. Once the pressures on opposite sides of the valve are equal, the valve will open since the areas exposed to pressure on the two sides are different.