We explore global variations in D' shear wave structure by examining 12 years of long-period Global Digital Seismograph Network (GDSN) data to identify reflected phases from the D' layer in the lowermost mantle. We restrict our search to epicentral distances between 63¿ and 74¿ where a precritical D' reflection will lie between the S and ScS arrivals. With GDSN long-period records a D' signal at these ranges is generally obscured by the stronger neighboring phases, so to isolate the D' reflections we develop a technique for stripping away the interfering S and ScS waveforms. We interpret travel time variations in the observed SdS phases in terms of variations in D' thickness and generate maps of inferred D' thickness for areas of good seismic coverage. The regions of observed D' reflections are beneath Australasia, north central Asia, the Arctic, Alaska, and central America. We find considerable variation in D' structure in these regions, a result consistent with previous studies which have found evidence for lateral heterogeneity in D'. On average, the estimated D' thickness is 260 km, but there is a fairly uniform distribution of thicknesses between 150 km and 350 km. A possible correlation is observed between D' thickness and regions of predicted strong horizontal mantle flow. ¿ American Geophysical Union 1994