We have used an optical pump-and-probe technique to measure the temperature dependence of the thermal conductivity,&kgr;(T),of isotopically pure Si. The sample was made from 99.7&percent;28Siby liquid phase epitaxy. Measurements were performed over the temperature range of 100–375 K. We found an increase in the thermal conductivity of isotopically pure Si, as compared to Si of natural isotopic abundance, throughout the entire temperature range. The results were theoretically reproduced by appropriately scaling the parameters used recently to fit the thermal conductivity of Ge samples with different isotopic compositions. A maximum in&kgr;(T)of∼4×104 W m−1 K−1is predicted for28SiatT≃33 K.©1997 American Institute of Physics.