Sound propagates differently and visibility varies according to the habitat type. Animals should therefore adapt the acoustic structure and the usage of their vocal signals to the environment. In the present study, we examined the influence of the habitat on the vocal behaviour of wild olive baboons (Papio hamadryas anubis) in two populations: one living in Gashaka-Gumti National Park, Nigeria, and the other in Budongo Forest, Uganda. We investigated whether female baboons modified the acoustic structure of their grunts and their rate of grunting when they wandered between closed and open habitat types. As an adaptation to the environmental conditions, baboons might utter calls with a longer duration, a lower fundamental frequency and/or energy concentrated in lower frequencies in a closed habitat like forest than in an open habitat. Baboons should also grunt more frequently in the closed habitat. Analyses showed that in both populations grunts uttered in forest were significantly longer than in open habitat. Additionally, baboons from Uganda showed a significantly higher grunt rate in forest than in open habitat. These results revealed a certain degree of plasticity in vocal production and call usage with regard to the habitat type. However, results in Nigeria suggested that, besides habitat structure, other proximate factors like the context of calling and the proximity between group members could also have an influence on the actual communication patterns.