Palaeoecological data from a small swamp in southern Ontario are used to reconstruct postglacial upland and wetland vegetation dynamics and assess the relative importance of autogenic and allogenic factors in wetland development. The inferred upland forest dynamics are comparable in timing and trend with lake records from the region. During the early Holocene, the shallow open water body became increasingly colonized by aquatic plants, until at around 8000 BP when the shallow open water community was replaced by a Larix swamp. At around 5600 BP, there was an apparent reversal in the wetland progression and Typha was locally dominant. There is a possible hiatus in the sedimentary record at around this time. Thereafter, a Thuj–Abies–Picea swamp community developed. The data suggest that internal factors and local events such as beaver activity could be equally or more important than regionally acting factors such as climatic change in controlling the hydroseral progression. The changing nature of the wetland community led to a 40-fold variation in sediment accumulation rate during the Holocene, emphasizing the importance of establishing a good chronology and using a multiple-core approach in such systems.