Northern pike, Esox lucius Linnaeus, collected from two regions in Ontario from 1961 to 1969, were studied to determine what characteristics and procedures might be used to externally separate the sexes. Spontaneous passage of reproductive products from the urogenital pore is the most accurate indicator of sex but can be used to differentiate such a small percentage of individuals annually that its value is limited. Even when stripped during the spawning period, only 82% of the mature males and 76% of the mature females yielded reproductive material. Differential body form alone is not a satisfactory indicator of sex. Sexing northern pike by the external appearance of the urogenital region is described. This technique can be applied to both mature and immature northern pike regardless of season. Accuracy of the method was tested throughout the year. Monthly averages of autopsied mature fish revealed that the urogenital technique was used to correctly sex 91% of the males and 94% of the females.