The study of the emerald shiner Notropis atherinoides Rafinesque was conducted in 1967 and 1968 in Lake Simcoe, a eutrophic inland lake with an area of 725 km2 situated in the Province of Ontario, Canada. On the basis of meristic characters, the emerald shiner of Lake Simcoe conforms to the classical description of the species and is not distinct from the form inhabiting the Great Lakes. Annulus formation occurred in early June on age I fish but second and third annuli were not formed on older fish until late July through August.Young-of-the-year emerald shiners reached a total length of 45 mm by October; age I were 81 mm and age II measured 92 mm by the same month. By September, age I female emerald shiners attained a size superiority of 5 mm over males of that age group and by the end of the third summer age II females exceeded age II males by 8 mm. Spawning occurred offshore in the latter part of July when surface water temperatures approximated 20°C. Of the three age groups, age II specimens were the most prevalent on the spawning grounds. Average absolute fecundity was 3410 eggs in fish of all age classes combined. Differences in water temperature seem to strongly influence the seasonal distribution pattern of the emerald shiner. The final temperature preferendum of the species, as determined in a vertical temperature gradient tank, was 25°C. The emerald shiner is extensively utilized as a bait fish by anglers and appears to be an important forage fish. The plasticity of the species to adapt to a variety of ecological conditions and the species potential for high production favour its introduction to waters requiring a good converter in the food chain.