Olfactory communication of the European otter Lutra 1. lutra was studied with particular reference to its role in the social organisation of the species. The study concentrated on the functions of scent marking for the otter and the types of information conveyed in the scent. Unlike other mustelids, otters do not scent mark directly with specialised skin glands. However, they deposit their faeces or spraints at prominent, traditionally-used places, which are subject to olfactory investigation. These sprainting places were distributed non-randomly in a coastal otter population. Spraints were aggregated at features of the environment used extensively by otters, e.g. small fresh-water pools, sleeping holes and rolling areas. Most thus occurred at the centres of activity within an otter's home range. These aggregations of spraints were spaced evenly along the coastline, as would be predicted if otters were maximising the likelihood of encounter of the odorous information. For most hypothesized functions of scent marking an animal must at least be able to discriminate the odour of its own marks from those of others. From conditioning experiments using a captive male otter, it was discovered that spraints carry precise, information on the identity of the depositor. In addition, all spraints from an individual can be associated by a common odour. Thus the potential exists for wild otters to monitor the movements of their neighbours by their contributions at sprainting places. Gas-liquid chromatography of spraints revealed that the individual differences in odour result from a complex blend of a large array of compounds present in subtly different concentrations. The sexual receptivity of female otters is probably also conveyed at sprainting places. The periodicity of oestrus was unknown for the European otter. Radioimmunoassays of urine revealed that females are probably continually polyoestrus, cycling with a periodicity of about 36 days. It is argued that the detailed individual identity inherent in the odour of spraints, and their spatial distribution in the field, do not support the popular belief that otters are territorial. Using evidence from other studies a new social system is proposed, based on widely overlapping home ranges and an absolute dominance hierarchy. Sprainting places provide the medium for constant monitoring of the movements and condition of other individuals.