Silicon is a common material in the solar system. For instance, Si accounts for about 10% of the material in primitive meteorites (CI chondrites). Since silicon is a refractory element, we expect the meteoritic isotopic composition to be very similar to that of the Sun. The isotopic composition of Si in meteorites is well known and varies little. Thus the three stable isotopes of Si may serve as powerful indicators to test fractionation of isotopes in the transition from the solar atmosphere into the solar wind. We present, for the first time, measurements of the isotopic composition of Si in the solar wind. The data were obtained with the MASS instrument aboard the WIND spacecraft and accumulated in exceedingly cold and slow wind. Such wind is often associated with large superradial expansion factors and with current sheet crossings which in turn are associated with the most efficient isotopic fractionation processes in the solar wind acceleration region. We detect little or no isotopic fractionation between the solar surface assumed to be of meteoritic composition and the solar wind. This constrains solar wind acceleration models and puts stringent limits on possible secular changes in the isotopic composition of the outer solar convective zone, the solar atmosphere, and the solar wind. ¿ 1998 American Geophysical Union