Differential travel times between S and SKS (Ts-sks), and SKKS and SKS (Tskks-sks), along with amplitude behavior have been used to study lateral variations at the core-mantle boundary. The data were recorded by the World Wide Seismographic Station Network (WWSSN) and the Canadian Seismograph Network (CSN) from source regions in South America and the Fiji-Tonga area. A core-mantle boundary region traversed by SKS and S beneath the mid-Pacific from the Fiji-Tonga area to North American stations shows the most anomalous behavior in Ts-sks. SKS and S are anomalously separated by up to 8 seconds. In addition to Ts-sks anomalies, SKS and SKKS are anomalous separated by 2 to 3 seconds. A model fitting Ts-sks for this anomalous region requires one or more of the following: (1) up to 5% increase in the top 300 km of outer core velocity; (2) a large scale high Vs region (2% increase) in the lower mantle NE of Tonga; (3) lateral variations in D' shear wave velocities of up to 3%. However, (1) above predicts a decrease in Tskks-sks, contradicting the observations. This suggests that (2) and (3) are more likely explanations in modeling this data set. ¿ American Geophysical Union 1988