The effects of tourism and human presence on the behavior and reproduction of pygmy marmosets Cebuella pygmaea, were evaluated in northeastern Ecuador. Field work was carried out from September 1996 to May 1998. Six groups of marmosets were observed in sites that differed in the number of tourists and use of motor boats. Reductions in social play and in the use of the lower stratum of the forests were significantly correlated with tourism pressure. Capture by local people of marmosets from two of the studied groups was associated with significant reductions in observability, vocalization rates and use of lower strata in these groups. All these behavioral changes may have allowed the marmosets to avoid contact with humans and were possibly related to differences in the reproductive performance of the groups. Recommendations are given for monitoring indicator species and for a better control of human activities in the area. (C) 2000 Elsevier Science Ltd.