The social structure and social interactions of the nocturnal slow loris Nycticebus coucang are described from locational as well as observational data on wild animals. Data were collected during 600 h of nocturnal radio-tracking and 400h spent radio-locating sleeping slow lorises during the day. Data are also presented on morphometrics, injuries and ectoparasites, and dispersal. The slow loris is generally described as solitary, but although close-range encounters were rare, animals were observed to form stable social units (‘spatial groups’) characterized by home-range overlap and friendly interactions among members and non-overlap between units. Four spatial groups were observed, each consisting of a single adult female, a single adult male and a varying number of younger individuals. Group composition, together with relatively small testis volume and natal dispersal occurring in both sexes, hints towards a monogamous mating system. One extended family group was formed by the delayed dispersal of a primary pair's offspring. We did not find another often quoted correlate of monogamy – a weak sexual size dimorphism. Friendly interactions among members of a spatial group included allogrooming, following, alternate click-calls, and sleeping in contact. Yet, members did not engage in any co-operative behaviour of the types usually thought to be responsible for group formation in gregarious mammals. One important factor contributing to the sharing of space between slow lorises is probably that chances of successful dispersal are low. However, subtle benefits arising from the presence of conspecifics (allogrooming, transfer of information on food resources) may also be crucial for the formation or maintenance of slow loris spatial groups.