Protective and territorial behavior was observed in 14 heterosexual pairs of adult siamangs in 11 zoos for a total of 1,155 h. The study shows that the quality of protective and territorial behavioral patterns was similar in the wild and in captivity. Under zoo conditions, the behavioral response (except calling) to conspecific and human rivals was similar. Males could be found more often at the front of the enclosure and were more active in protective and territorial behavior than were females. Males were more attentive to happenings outside their enclosures than were their mates, whereas females concentrated their protective and territorial activities on specific people or females of a conspecific group. Although the duration of each song and the average number of duet sequences as well as the temporal distribution of calling throughout the day were similar in the wild and in the zoo, the total duetting rate differed remarkably: it was much higher in captivity. Siamangs in acoustical and visual contact with neighboring conspecifics spent more time singing than did siamangs without such contacts. In captivity, pairs without young seemed to be more engaged in protective and territorial behavior patterns than were parents. Zoo Biol 16:309-325, 1997.