In response to concerns that the environmental quality of Lake Couchiching is under pressure from increasing recreational uses and development along its shores, this study was conducted to: (i) establish a baseline database with respect to the lake's limnology, water, sediment and biological quality, (ii) define the existing aquatic environmental quality and trophic status of Lake Couchiching (i.e., at the present level of development); and, (iii) develop recommendations concerning possible impacts associated with future development. Based on these overall objectives, this study characterized water and sediment quality at several locations in Lake Couchiching. This study also characterized bacteria, phytoplankton, zooplankton, macroflora (macroalgae and macrophytes) and benthic macroinvertebrate communities. This study did not attempt to characterize the condition offish populations in Lake Couchiching since the local Ministry of Natural Resources offices routinely update that information. The biological communities (phytoplankton, zooplankton, macroflora, and benthos) that are characterized in this study are however, capable of being used as early warning indicators of potential effects on fish populations in the lake. Surveys were conducted throughout the ice-free period in 1997.