In Britain, the water vole (Arvicola terrestris) has declined in numbers and distribution (Jefferies, Morris & Mulleneux, 1989; Strachan & Jefferies, 1993). This species now enjoys a higher profile than hitherto and many local surveys have been initiated, following published methods. To monitor the status of the population or the effectiveness of habitat restoration projects, a method for estimating numbers of water voles is often needed, preferably based on latrines (a characteristic field sign of this species). Unfortunately, the published literature includes two potentially misleading components. In view of the significance of population size in a declining species, and the heightened interest in this animal, we felt it would be helpful to publish clarification and corrections in the hope of forestalling erroneous reports and conclusions in future investigations.