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Data (2083 sites visited) collected during the otter Lutra lutra (Linnaeus, 1758) survey of Poland in 1991-1994 were used to evaluate some of the factors affecting to efficiency of the technique. The success in detecting otter signs was found to be significantly affected by the presence of "spraintable" bridges and other potential sprainting sites on the banks. Decreased efficiency of the technique was noted in specific habitats, eg large rivers, canals surrounded by open fields or meadows, and any aquatic habitats with few potential sprainting sites on the banks. It is suggested that modifications: spot checks at additional bridges and extended searches of river banks, could be used to improve the reliability of the survey in areas with low numbers of otter signs.