The ability to recognize and remember individual identities for long periods of time has important implications for the evolution of animal social behaviour, particularly complex interactions such as cooperation or mate choice. Despite this importance, there is only a single example of long-term individual recognition in nature, the 8-month retention of neighbour's song among male hooded warblers, Wilsonia citrina, and there is none for a non-human mammal. Associations between individuals spanning years, which are especially prevalent in carnivores, primates and seabirds, and evidence of mate fidelity provide indirect support for the ability of long-term recognition. In many of these instances, however, individuals do not separate for extended periods, and thus long-term recognition, although often assumed, may be both unnecessary and nonexistent. Furthermore, site fidelity rather than individual recognition may explain many instances of mate fidelity. Here I show that mother-offspring pairs of a migratory otariid pinniped--the northern fur seal (Callorhinus ursinus)--not only have the ability to recognize each other's vocalizations during the course of a breeding season, but are also able to retain these memories for at least 4 years.