The first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported. Both marine and riverine d30Si are more positive than d30Si of igneous rocks, suggesting isotopic fractionation during weathering and clay formation and/or biomineralization. The d30Si value for dissolved silicic acid from several ocean basins is 11.1 6 0.3‰ (n 5 69). More positive values are found in surface waters, due to discrimination against 30Si during biogenic opal formation by diatoms. The deep Pacific Ocean is depleted in 30Si relative to the deep Atlantic by roughly 0.4‰. d30Si of riverine dissolved silicon averages 10.8 6 0.3 (n 5 8). The biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea. The average d30Si value of inputs of silicon to the ocean most likely falls between 10.3 and 10.9‰, and that of the outputs falls between 10.9 to 11.9‰, probably in the neighborhood of 11.0‰.