The life history of eels in Lake Ontario was studied by measuring, tagging, and releasing about 1500 individuals between 1958 and 1966. The smallest eels captured were from 295 to 318 mm long. Presumably they entered the lake at about age III–IV at lengths of 200–300 mm. Otoliths of eels from the lake showed a range in age between IV and XVIII, while those of smaller eels from the Ottawa River ranged between age II and IX. The length–weight regressions showed two distinct growth phases: one from about 480 mm to 790 mm, the other from about 830 mm to 1080 mm. Growth of the orbital diameter, interorbital distance, and pectoral fin length in relation to total length was not obviously related to sexual maturity. Eels that emigrated from the lake had a greater weight–length ratio than those that remained in the lake for another year. Of 917 tagged eels transplanted within Lake Ontario, 28 were recaptured near and 18 recaptured far from the original capture site. Although the results are inconclusive, there is a suggestion that eels have home territories within the lake.