During a nine month field study, we assess the ability of spider (Ateles geoffroyi) and howler (Alouatta pigra) monkeys to cross a large Mesoamerican river, and if this behavior is related to deforestation and/or human population size on the disturbed riverbank. The study was conducted along the Lacantún River, southern Mexico, which .divides the Montes Azules Biosphere Reserve and the Marqués de Comillas disturbed region. We collected data on river crossings by both monkey species from 428 hours of surveys along the river and 58 questionnaires completed by knowledgeable local informants. Furthermore, we determined the frequency of river crossing by the two species, location and direction of river-crossing, bank-to-bank river width at the location of reported sighting, and the relationship with deforestation rates and/or human population size in the Marqués de Comillas region. We observed on two separate occasions spider monkeys crossing the river from the disturbed area to the reserve, but howler monkeys were never observed crossing. Interviews suggest, however, that howler monkeys cross the river more frequently than spider monkeys (13 versus 8 reports, respectively). We found that bank-to-bank river width was smaller at river crossing locations in each study site than along the river in general. Furthermore, we found no relationship between river crossings and some human pressures such as deforestation and population size in the region. Whatever the underlying pressures that drive river-crossing events in these two primate species in this region, they appear to be sufficiently strong to stimulate this behavior to occur from time to time.