1. Moustached tamarins, Saguinus mystax, are frequently living in mixed-species troops (associations) with saddle-back tamarins, Saguinus fuscicollis .
2. In a mixed-species troop of these two species, the moustached tamarins spent 82% of the activity time in association with the saddle-back tamarins. Degree of association varied during daytime and between days.
3. Interspecific social interactions were observed at a rate of one per 5 h of observation and were mostly agonistic. The majority of agonistic interactions occurred at small food resources. Moustached tamarins always dominated saddle-back tamarins. Friendly interactions were extremely rare.
4. Mutual calling by means of long calls was observed in the morning after leaving the sleeping sites and during the day when the species became separated. This indicates that mutual attraction exists between moustached and saddle-back tamarins.
5. Interspecific interactions and mutual calling incur some costs of living in mixed-species troops. The rareness of interactions and the low rate of interspecific calling compared to Intraspecific calling may hold these costs relatively low.
