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This study reports on the ecology of the Belizean black howler monkey (Alouatta pigra) in two different habitats. Monkey River is a riparian secondary forest whereas Runaway Creek Nature Reserve (RCNR) is a primary and mature forest situated in a limestone karst hills landscape. This type of ecosystem, neither the population inhabiting this reserve, has been studied before. We contrasted food availability, diet, group size and composition, population density, home range size and activity patterns between those two populations. We predicted the disturbed riparian forest to have higher food availability but a less diverse diet with a higher consumption of fruits. Thus, we predicted howlers to have higher population density, larger groups with more males and more infants, smaller home ranges with more overlapping. Also, we predicted activity budget to be biased toward a less active lifestyle with less travel but more inactivity, and more social interactions. Our results confirm some of those predictions as food availability is higher in Monkey River with food species accounting for 80% of the diet and all food species of howlers diet having a higher total relative basal area. This is likely to be associated with the higher population density (44.82ind/km² in Monkey River against 26in/km² in RCNR) and smaller average home range size (3.27 ha against 11.87 ha) with a higher proportion of overlapping (11.87% against 0%). Predictions on group size and composition are not confirmed as the difference in mean group size is not statistically significant and as many males per group are found in both habitats (one) but sex ratios (M:F) indicate the presence of more females in Monkey River (1:1.6 against 1:1.3). Also, more infants per group are found in RCNR (0.6 in Monkey River against 1 in RCNR). Those results are likely to be associated with different stage of population growth between the two habitats and more precisely of the hurricane Iris that have lowered the population in Monkey River and allowed more dispersal opportunities and, resulting effects of social factors such as infanticide. Nevertheless, our results indicate howler population to increase again in this disturbed forest. Howlers in RCNR have a more diverse diet (18 food sources in Monkey River against 23 in RCNR) which is likely to be due to higher diversity of plants present in the limestone karst hills. Diets in both habitats differ as only 19.5% of species are similar and species composition in both habitats are pretty different too, which confirms howlers having a flexible diet and being able to adapt their diet to the species found in the habitat. Both populations feed preferentially on leaves but howlers in the secondary forest spent more time feeding on fruits (20.46%) and less on flowers (6.46%) than in the primary forest (11% and 11.75% respectively), although those differences are not significant. Howlers in Monkey River are more active and travel significantly more (9.45% against 5.45%) which is likely to be due to the higher amount of fruits in the diet. Less time is spent in social interactions in Monkey River, which is likely to be due to the smaller number of infants per group. Finally, monkeys in the secondary forest spent significantly more time vocalizing than in the primary forest, which is likely to be due to the higher population density and level of overlap between neighboring groups. No overlap has been recorded in the limestone karst hills and percentage of vocalization is quite low.