Range and average values were determined for the total and free form levels of the serum vitamin B₁₂ of the mouse, rat, guinea pig, rabbit, dog, horse, cow and calf. Liver vitamin B₁₂ levels of these animal species were also determined except the horse, cow and calf. The total B₁₂ levels of the serum had a wide variation among the various animal species. The species with the highest total B₁₂ level of the serum was the rabbit and the level was about one hundred times higher than that of human serum, which was lowest among the sera of the various species examined. The ratio of the free form to the total B₁₂ varied also widely among the species. The serum B₁₂ of the various species was classified by the mean ratios of the free form to the total into three groups. The liver vitamin B₁₂ levels of various species did not show such a remarkable diversity in contrast to the serum B₁₂ levels.