Reavie and Smol present the first detailed description of the diatom flora of the St. Lawrence River, Canada. They provide light micrographs of 227 diatom taxa from 35 genera, recorded in 299 samples from the freshwater portion of the St. Lawrence River, between Kingston and Quebec City. Diatoms were identified in both plankton samples and periphytic collections (rocks, macrophytes, filamentous green algae) as well as sediment core samples. High species diversity is observed so that the discussion was limited to the more common taxa (that is taxa that occured in at least three samples, or had a relative abundance of more than 1% in one sample). The taxonomic and iconographic sections of this book include descriptions and illustrations of the taxa encountered, as well as autecological information. These data may assist future biomonitoring and paleolimnological study efforts.