Electron microscopy of silica scales was used in the identification of 13 taxa of the colonial chrysophyte Synura found in Ontario lakes and ponds, one of which has not been reported previously from North America (Synura spinosa f. longispina Petersen & Hansen). Two other taxa found in Ontario (Synura splendida Korshikov and Synura petersenii f. praefracta Asmund) have previously been reported only once from North America. One new combination is proposed (Synura leptorrhabda (Asmund) Nicholls) which provides for more consistent taxonomy of species in the Synura echinulata group (S. echinulata Korshikov, Synura mammillosa Takahashi, Synura multidentata (Balonov & Kuzmin) Péterfi & Momeu, and S. leptorrhabda). The systematics of other problematic taxa, including the related genus Chrysodidymus, are discussed. Samples containing Synura spp., previously implicated in 11 recent cases of "fishy" taste and odour in Ontario lakes, were reexamined by electron microscopy and found to be dominated by S. petersenii Korshikov. All affected sites were lakes of the Precambrian Shield (pH 5.7 – 7.5; alkalinity < 1 – 45 mg CaCO3/L). No evidence of obnoxious tastes or odours from other Synura species has been found.
