The Grande-Plée-Bleue peatland in southern Quebec is the second largest undisturbed ombrotrophic peat bog south of the St. Lawrence River. Macrofossil analysis of 2 sediment cores was performed to reconstruct the developmental stages (flora, trophic stages) of a sector of this ecosystem located in a peri-urban area. Organic accumulation began around 9500 cal. y BP (calibrated years before present) in one or several small, shallow ponds after the retreat of the postglacial Goldthwait Sea. The spatial growth of the peatland took place through paludification to the detriment of a forest that was initially composed of balsam fir and white birch. Ombrotrophic conditions formed very early in the history of the site, around 8300 cal. y BP. Since that time, the bog's vegetation cover (composition, structure) has been quite stable. The longterm dynamics of the peatland was controlled by autogenous factors. However, more humid conditions were present locally between ca 1050 and ca 400 cal. y BP Similar indications have also been found in other bogs of southern Quebec for the same period, which suggests a change in regional hydroclimatic conditions.