A brood of 45 fish divided into 2 sets and reared at different temp. levels were measured at monthly intervals. 2 smaller broods maintained for shorter periods were given a somewhat different diet and measured. Using a method of describing growth characteristics as regression of length at age n + 1 against length at age [image] the rate of deceleration of growth is shown by the slope of this "transformed" growth curve. For this genetic strain the rate of deceleration is relatively constant both for different metabolic rates, as under slightly different temp. conditions, and in different sexes and probably at different levels of food intake as well. This growth characteristic may be useful as a physiological character of genotypic importance while differences in level of transformation lines probably represent phenotypic variation.