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 for other healthy Margaritifera populations. 6. Only two live juveniles were found. However, this is likely to be due to the survey being restricted to counts of visible mussels only, and the age structure being based on the analysis of dead shells. 7. The population's current status and possible reasons for its survival in this river are discussed. Conservation measures should include the construction of a fish ladder to make fish migration through the culvert possible, removal of a metal screen preventing fish migration from the upper reaches of the river to the lake, reduction of recreation activities, and providing the local children's camp with water treatment facilities.