Recent studies indicate that many of the nocturnal prosimian primates are gregarious rather than solitary. This paper shows that the spectral tarsier is gregarious during its nightly activity period as well as in its sleeping tree. Using mist nets and radiotelemetry, focal follows were conducted on six groups at Tangkoko Nature Reserve in Sulawesi, Indonesia. During 442 focal follows, 1072 encounters between a focal adult group member and another adult were observed. The number of encounters ranged from as few as 0 to as many as 18 encounters per night. Intragroup encounters lasted from less than 1 min to as long as 3 hr 12 min. Nearly one-half of all social behavior occurred between adult females and males. There were also substantial rates of social interaction between the two adult females in one group, and between sub-adults of the opposite sex in neighboring groups.