1. An internationally important population of the freshwater pearl mussel (Margaritifera margaritifera) was rediscovered in a small river in north-west Russia. 2. A survey of visible mussels in 2008/2009 indicated an estimated population of 40 000 individuals. This is the largest population currently known in the Leningrad oblast and is comparable with the entire pearl mussel population of some western European countries. 3. The average density of visible mussels was 29.6 individuals m−2 in the middle part of the river. In the four largest mussel beds maximum densities of 1000+ individuals m−2 were recorded. Such densities are exceptional and have not been reported elsewhere in Europe during the last 100 years. 4. Live juvenile mussels were recorded, indicating that that this population is viable, although further investigation is required to establish its status. 5. Analysis of the population age structure, based on the measurement of empty shells, showed an age class distribution similar to those reported f