Loud calls can be expected to play an important role in the lives of howler monkeys, given the specialised anatomy of howler vocal apparatus and the time and energy invested in calling. Here we present observational and experimental data aimed at understanding the function(s) of the roars of black howler monkeys (Alouatta caraya). Most roars were given spontaneously, especially around dawn, although inter-group encounters and extreme weather events triggered calling. Roars were given throughout the home range, but not uniformly; variations in calling frequency with location were not well predicted by frequency of use, and calling was not more frequent at borders. Predator presence was neither necessary nor sufficient to stimulate calling. We experimentally played back loud calls from stranger groups, either inside the home range of the study group, simulating invasions, or in border areas. In response to simulated invasions, the alpha male roared more frequently than expected, usually in the vicinity of the playback site, moved off sooner and travelled to or near to the playback site. When playback was in border areas, the alpha male roared infrequently and significantly later, and did not travel towards the playback site. These results are not consistent with the hypotheses that roaring functions in predation deterrence and/or mate defence. Instead, they suggest that roaring allows regulation of the space use, by means of regular advertisement of occupancy but not by mutual avoidance or boundary defence. We believe that roars also provide a mechanism for reinforcing occupancy during encounters, and may sometimes serve to settle disputes without chases and fights.