Fire is an important mechanism of disturbance in boreal ecosystems; however, the effects of fire on lake ecosystems are still not well understood. This study provides a detailed assessment of the impacts of fire on the limnology of a small oligotrophic lake (Lake 42), located approximately 200 km northwest of Thunder Bay, Ont. The study lake is characterized by a small drainage ratio (watershed area: surface area) and a relatively long water residence time. Age establishment and fire scar analyses determined that at least one, and perhaps two, major fires had burned to the lake's shoreline in the past ca. 150 years. Using a paleoecological approach, diatoms were examined in a 210Pb-dated sediment core. Following watershed fires, minimal changes were noted in the diatom species assemblage. These findings may be explained by the low sedimentation rates and small drainage ratio of the study lake, although other studies suggest that the biological response may be minimal compared with physical-chemical responses in some ecosystems. Beginning in the early 1980s, however, distinct changes were noted in the species assemblage and in diatom-inferred total phosphorus. Our findings suggest that the study lake may be more sensitive to precipitation inputs of nutrients than to inputs resulting from watershed disturbances.