Seismic array recordings are used to study the heterogeneity of a 15¿ ¿ 25¿ region of the lowermost mantle beneath the northern Pacific. We investigate P waves from northwestern Pacific events, 68¿ to 82¿ from the Yellowknife array in northern Canada. Anomalous arrivals (PdP) are observed 2--13.5 s after P with a slowness 0.4--1.2 s/deg smaller than P, suggesting that they are reflections from a D″ discontinuity. We use vespagrams (slant stacks) and f-k analyses to determine travel times and slowness vectors respectively. The f-k technique simultaneously estimates both the horizontal slowness and backazimuth of arrivals at a receiver, with better resolution than vespagrams. Travel time analysis reveals a mean discontinuity height of 241 km above the core-mantle boundary in the region studied here. However, there appears a systematic variation in this thickness which ranges from 211 to 336 km. The f-k analyses also reveal variations between P and PdP backazimuths, further implying the existence of lateral variations in D″ in this area. This site is thought to be a transition region from an area of mantle downwelling to a region of upwelling; thus the variations in heterogeneity in this region may be related to its proximity to a site of paleoslab accumulation.