The East Bay and Castle Bay stratigraphic sections along the northeastern shore of Bras d'Or Lake, despite some discontinuities, represent an important part of the Upper Pleistocene. Pollen analysis of these deposits provides a record that can be subdivided into three main units: (1) The first unit reflects the spread of rich forests dominated by thermophilous species such as Quercus, Ostrya, and Pinus strobus. It indicates a warmer and certainly drier climate than the present one. Th/U ages of the order of 120 000 years support a correlation of unit I with the Sangamonian (sensu stricto) interglacial (substage 5 in oceanic climato-stratigraphy). (2) The lower part of unit II, characterized by Abies balsamea, Fagus grandifolia, Tsuga canadensis, and Pinus, reflects the predominance of a mixed forest in the regional landscape. A cool and humid climate, probably not dissimilar to that of present is inferred. Th/U ages of approximately 85 000 years permits correlation of this forest stage with a relatively warm interval of the last interglacial, probably substage 5a. The upper part of unit II reflects a cooler climate and is characterized by pollen assemblages of the coniferous species Abies balsamea and Picea. (3) The lower stratigraphic part of unit III represents the spreading of coniferous boreal forests dominated by Picea, Pinus, and Abies during a cooler interval than at present. An approximate Th/U age of 60 000 years was obtained. Unit III reflects several cycles interpreted as alternating successions of tundra–forest and coniferous boreal forest. It suggests minor climatic fluctuations during a relatively cold interval. The third polynostratigraphic unit covers a significant part of the middle Wisconsinan (stage 3).