The isotopic composition of steam from the Larderello, Italy, and The Geysers, California, geothermal fields is used to determine the source(s) of the fluid in these two vapour-dominated systems. Previous interpretations suggested the isotopic composition of the two systems was mainly the result of reactions at high temperature between deeply circulating meteoric water and largely sedimentary host rocks. We interpret the data for the Larderello and The Geysers fluids as indicating that meteoric water, exchanged with host rocks, mixes with local magmatic water. The isotopic composition of end-member magmatic water at The Geysers is typical of convergent plate boundaries (deltaO-18 = +5 to +11 per mil; deltaD = -10 to -35 per mil); a local isotopic composition of +11 to +15 per mil deltaO-18 and -15 to -35 per mil deltaD is suggested for the Larderello magmatic water. The magmatic water derives from the crystallization of underlying magma. Metamorphic waters, derived from dehydration reactions of OH-bearing minerals, may also make a minor contribution to the geothermal fluids.