Throughout Alberta and Saskatchewan, the upper Albian Substage lies unconformably on subjacent beds. However, on the Peace River in northeastern British Columbia, 32 km below Hudson Hope, a silty shale outcrop shows uninterrupted marine deposition across the Middle–Upper Albian substage boundary. An arenaceous foraminiferal fauna of 21 genera and 57 sub-generic taxa is illustrated from this outcrop of the Hasler Shale. The collections come from above the previously illustrated microfauna of the Stelckiceras liardense Zone and below the occurrence of Haplophragmoides gigas gigas and provide a spectrum of transitional faunules spanning the middle–upper Albian boundary. The corresponding boundary of the Ammobaculites wenonahae Subzone of the Gaudryina nanushukensis Zone with the overlying Haplophragmoides uniorbis Subzone (new) at the base of the Haplophragmoides gigas Zone is defined at 73 m above the top of the Cadotte Sandstone by the disappearance of A. wenonahae. Genera within the upper part of the A. wenonahae Subzone and the H. uniorbis Subzone include Bathysiphon, Saccammina, Pelosina, Hippocrepina, Psammosphaera, Thuramminoides, Ammodiscus, Miliammina, Psamminopelta, Reophax, Scherochorella, Haplophragmoides, Ammobaculites, Bulbophragmium, Ammobaculoides, Textulariopsis, Pseudobolivina, Trochammina, Gravellina, Eggerella, and Verneuilinoides. Two new species and two new subspecies are established, viz. Ammobaculites kokei, Trochammina bredini, Haplophragmoides gigas Cushman pregigas, and Reophax deckeri Tappan sliteri. Species originally described from the upper Albian of Wyoming, such as Haplophragmoides uniorbis Eicher, originated out of boreal stock, as they are recognized within the middle Albian upper A. wenonahae Subzone north of the Peace River Arch. On the Peace River, the passage from middle Albian to upper Albian is marked by water depth at or below wave base, but regression is reflected at the substage boundary by the sharp reduction in variety of taxa.