The first account is given of the postglacial history of a lake in eastern Canada. Splan Lake is a 4 ha lake situated in a hollow in Palaeozoic metasedimentary rock in southern New Brunswick. Diatoms were identified and counted in a 6.98 m core, from which five developmental phases were recognized. Initially Splan Lake supported a pioneer Fragilaria assemblage comparable to that in modern arctic and glacial moraine lakes. This is believed to represent growth in the moat of a partially ice covered lake. An embryonic limnic flora consisting of Cyclotella bodanica and C. stelligera appeared ca. 11 300 y.B.P., which was terminated by the younger Dryas cool intervalca. 11000-10000 y.B.P. There appeared to be no autochthonous production in Splan Lake during this latter episode. Subsequently, a C. bodanica: C. stelligera: Navicula community developed. From ca. 8 800-5 500 y.B.P. Asterionella ralfsii v. americana, Tabellaria spp. and large numbers ofchrysophyte scales occurred together with Cyclotella spp. The 'recent' phase is dominated by Tabellaria, Frustulia, Fragilaria, Eunotia and Navicula. The lake evolved from alkaline to slightly acid, and from oligotrophic to mesotrophic following the younger Dryas. The flora developed from benthic/littoral to a predominantly littoral/limnic community over the same period, and with continued sedimentation, into a littoral/benthic diatom community.