<p>The activity patterns and feeding habits of primates are altered in response to habitat degradation and fluctuations in food availability. Among the Old-World monkey species, the De Brazza’s monkey (DM), Cercopithecus neglectus, is one of the understudied species in Africa. We evaluated the impact of habitat disturbance on activity patterns and diet of the DM in the three management focal zones (core, buffer and transition) of Bonga and Saja forests of the Kafa Biosphere Reserve. Data were collected using instantaneous scan sampling method from June 2022 to May 2023. Six groups of De Brazza’s monkeys (DMs)- one group from each management zone of the two forests- were observed twice a month. The proportion of time spent on each activity was calculated by dividing the number of records for each activity by the total number of activity records. Slightly more behavioral activities were recorded during the dry season (50.07%, n = 3173) compared to the wet season (49.93%, n = 3163). Feeding was more prevalent in the buffer zone (34.9%, n = 713) compared to the transition zone (31.6%, n = 646). In contrast, moving was most common in the transition zone (38.9%, n = 441) and less frequent in the buffer zone (24.9%, n = 333). The study revealed that the diet of De Brazza’s monkeys consists of 34 plant species, 27 at Bonga and 24 at Saja site, with 17 species common to both sites, while 10 were unique to Bonga and 7 species to Saja. Most of the feeding activity was relatively higher during the wet season (51.8%, n = 2,124). Majority (83.7%) of the feeding scans were scans on fruit bearing trees. More DM diet sources were recorded in transition zones of the two study sites. The findings indicate that moving was more frequent in disturbed zones than the others zones. Seasonality influences the behavior of De Brazza’s monkeys, with a greater proportion of their activities observed during the dry season. An immediate conservation action is necessary to mitigate habitat degradation in the Biosphere.</p>