The yellow lampmussel (Lampsilis cariosa) has been unofficially listed as threatened in the United States and Canada and as endangered globally by the IUCN. Although it has been suggested that it may be extirpated in New Brunswick, we show that a large and historically well-established population of L. cariosa persists in the lower Saint John River and tributaries. This is in spite of evidence that the species range in the lower Saint John system has contracted over the past century. Although further work is needed to estimate overall abundance of L. cariosa in New Brunswick, the lower Saint John and tributaries appear to harbor most of the known Canadian population. The apparent size and presumed genetic integrity of the isolated New Brunswick population, as well as the current lack of zebra mussels (Dreissena polymorpha) in the Saint John system, suggests that this population could play an important role in the future conservation of the species.