Broad-band data from the Canadian National Seismograph Network (CNSN) shows evidence for a laterally-varying S-wave discontinuity in the lowermost mantle beneath the Caribbean. The magnitude of the velocity contrast across this D″ discontinuity varies inversely with its distance from the core-mantle boundary (CMB). Clear evidence for a 2.75% increase in shear velocity 250 km above the CMB is observed in a region 10 ¿N and 60--85 ¿W; a thicker D″ layer (290 km) with a less pronounced topside discontinuity (2.45%) is interpreted in the region 80¿--90 ¿W and 20 ¿N; and there is no evidence for a D″ discontinuity in the region 65¿--80 ¿W and 25 ¿N. Considerable 3D structure within the D″ region is suggested by travel-time scatter in ScS arrivals and S arrivals which turn within D″. Transverse-component SKS signals due to upper-mantle anisotropy can be confused with D″ signals; however, we show that false interpretations of D″ structure can be mitigated through the careful analysis of travel-time moveout and low-pass filtering. ¿ American Geophysical Union 1996