A social group of 37 mandrills, with composition resembling a wild group, maintained in a 5.3 ha enclosure of natural relict gallery forest at CIRMF, Gabon was studied over 29 months. The 14 wild-caught founders (7 adult females, 2 adult and 5 subadult males) were the subjects of detailed behavioural study (15 months, 1200+ hours observation). Mandrills were captured at least annually to obtain blood samples, data on body weight, dental and reproductive status, and testicular volume. Breeding was seasonal, with a 4-month mating season in which 92% of oestrous periods occurred. Oestrus synchrony was evident, with up to 5 females maximally swollen on any one day. Most (92%) copulations occurred at maximum swelling, with ejaculation in a single mount. 'Mate-guarding' of pen-ovulatory females by the alpha male involved sustained proximity to her and 'warning grunts' to other males. No herding behaviour was observed. Spatial, grooming, and agonistic relationships were examined in detail. During anoestrus, males were rarely near females; three males were never recorded allogrooming. The alpha male spent significantly more time close to anoestrous females, grooming with them significantly more than the other males. Females spent time near each other, groomed with their offspring, and gave three-quarters of their grooming to and received nearly all grooming from founder females. During oestrus, male-female proximity increased, females spent more time grooming, groomed with fewer age-sex classes, groomed mostly with male founders, and received more grooming from males. Stable, linear dominance hierarchies existed within each sex. Various aspects of mandrill biology - colouration, scent-marking, vocalisations - were interpreted as adaptations to ecological constraints of living semi-terrestrially in tropical forest. Results were used to assess models of single- and multi-male social organisation and male mating strategies. It was suggested that mandrills form one-male units, different from those of hamadryas and gelada baboons. Instead similarities with an Asian ecological analogue of the mandrill, the pig-tailed macaque, were emphasised.