Differential PKP AB--DF travel-time measurements are compared directly with predicted residuals for mantle tomographic models to investigate the heterogeneity of the lowermost mantle. Much of the AB--DF time is accumulated in the lowermost portions of the mantle, resulting in heavy weighting of 3-D lowermost mantle structure on the differential travel-time perturbations. Comparisons of the observations with tomographic models show strong model dependence. The best variance reductions are achieved using the recent P-wave model of van der Hilst et al. [1997> (up to 20%), but the heterogeneity in the lowermost mantle may be 8 times larger than what shown by the tomographic model. The predictions for present S-wave tomographic models correlate poorly with the observations if we assume P-velocity anomalies are proportional to S-velocity anomalies. We argue that S-velocity anomalies in the lowermost mantle do not always track P-velocity anomalies. Our study shows no obvious evidence of slab effect on the observed AB-DF anomalies and, thus, the use of PKP phases is likely to provide a powerful tool in mapping the P-velocity structure in the lowermost mantle.¿ 1997 American Geophysical Union