Serial stable isotope analyses (carbon and nitrogen) of collagen from the tusk of a juvenile (5.5-6.0 years of age) woolly mammoth from Wrangel Island reveal a long-term trend toward less positive δ15N values. This is the trend expected for an animal decreasing its dependence on its mother's milk. In addition to the long-term trend, there are in-phase seasonal oscillations in both δ13C and δ15N. These oscillations are hypothesized to be the result of seasonal fluctuations in the proportion of plant protein in the diet. During the short arctic growing season, the calf appears to have supplemented its diet with plant proteins (more than at other times of year) leading to more negative δ13C values and less positive δ15N values. Unfortunately, this animal died before the declining trend in δ15N associated with weaning clearly leveled off. Thus, we cannot be certain that the weaning process was complete at the time of death. This animal's tusk record nonetheless suffices to establish a lower limit on weaning age of about 5 years, comparable to the weaning age of African elephants in high-stress environments.