The European species of Salmo have fewer vertebrae than those from western North America, the mean values of two of each group as reared at the Cowichan hatchery being found to be: salar 59.0, trutta 58.3, gairdnerii 63.5, clarkii 62.5. Hatchery-reared gairdnerii were usually found to be different from those of the natural environment as well as varying with different experimental conditions, and even differed from their own parents, this seeming on the whole to be related to the temperature during development. A correlation was found between the length of the fish and the number of vertebrae. Caution is suggested in the use of the character for identifying populations of Salmo.