We present ultrahigh precision measurements of the optical heterogeneity of high quality fused silica at the &dgr;n=10−8index level. The measurements were made with a unique scanning heterodyne interferometer developed as part of the Gravity Probe‐B Program. Striae of refractive index variation &Dgr;n<10−7(optical path difference smaller than 5 nm) are clearly visible on an extended heterogeneity background of around &Dgr;n=6×10−7. Both striae and extended heterogeneities are repeatable to within a few parts in 108. These results show the nature and extent of the heterogeneities of high quality vitreous silica. ©1995 American Institute of Physics.