An investigation was made of the diatom stratigraphy in a sediment core from Sunfish Lake, a small (8.3 ha), deep (20 m), meromictic lake in southwestern Ontario. About 340 diatom species and varieties distributed among 37 genera were identified from the sediments; however, only 16 taxa had more than 2% representation in any sample. On the whole, the number of individuals and diversity of the samples increase over the postglacial period. Diatoms first appear in pollen zone A and consist mainly of the littoral forms Cymbella diluviana, Fragilaria lapponica, and F. construens var. venter. It is suggested that these taxa are typical of early postglacial lacustrine sediments in northeastern North America. Cyclotella bodanica first appears in zone B and is dominant in most samples throughout the core. It is euplanktonic and typical of subalpine, circumneutral, oligotrophic lakes. In C2, when the climate was possibly warmer and drier than present, there is evidence for a lower lake level: C. bodanica is temporarily replaced in importance in the sediments by C. kuetzingiana and the proportion of littoral and epiphytic diatoms increases. Cyclotella bodanica regains its former importance in C3 sediments with the possible return to a cooler, wetter climate. About 850 years before present (B.P.) the lake evidently became eutrophic. Cyclotella bodanica declines and there is an increase in the proportion of several species typical of a higher trophic level, e.g., Stephanodiscus hantzschii, Cyclotella glomerata, and several Synedra spp. The cause is unknown. The lake probably became meromictic about 140 B.P. at a time when the forest was cleared and agriculture began in the watershed.