Eggs of the brook trout and the rainbow trout were incubated at various constant levels of temperature and dissolved oxygen to test the hypothesis that vertebral number in fish is controlled, within limits, by the developmental rate. The mean vertebral number in both species decreased with increased rate of development which was caused by higher temperature and conversely the mean vertebral number in both species increased with decreased rate of development. The number of vertebrae is shown to be inversely related to the rate of development, expressed as the reciprocal of the number of days in the period of numerical determination of vertebrae in each species. The slopes of the regressions are −0.45 and −1.15 for brook trout and rainbow trout, respectively, and are perhaps, characteristic of the species. Variations in the usual type of trend of vertebral number with different developmental conditions, as reported by some investigators, are discussed and a possible explanation for some of these variations is proposed.