Sometimes referred to as sea unicorns, narwhals (Monodon monoceros) are Arctic cetaceans which throughout history have been revered as unique and mystical animals. In pre-nineteenth century societies, the long, up to 2 m in length, counterclockwise spiralling tooth of the male narwhal was thought to be the ‘tusk’ or alicorn of the legendary unicorn1,2. Alicorns were also thought1,2 to possess a variety of powers such as being able to detect and prevent poisoning. During the fifteenth and sixteenth centuries, narwhal tusks had a value of ∼10 times their weight in gold3, and thus they were often used to settle debts. Extensive hunting by seventeenth–twentieth century European and American whalers probably caused short-term reductions, but apparently no long-term effects, in the narwhal population4. Only Eskimos continue to hunt narwhals mainly for their tusks, although they also consider the skin (muktuk) a delicacy3. Many recent studies of narwhals have been concerned with their distribution, abundance, life cycle, and behaviour4–11 in the hope of aiding the management of this exploited species. Yet little is known about the population dynamics of narwhals, because determining the age of any particular animal is difficult. We report here that the extent of aspartic acid racemization in narwhal teeth provides a means for determining the age of narwhals. This method may be particularly useful for mature females, the ages of which have previously been difficult or impossible to determine.