Conservationists are missing opportunities to protect species at mass tourism sites where wildlife itself is not the main tourist attraction. At such locations are ‘incidental ecotourists’, i.e. tourists with multiple interests who encounter wildlife or fragile ecosystems inadvertently. A case study from Lamanai Archaeological Reserve, Belize, reveals the motivations of incidental ecotourists and their impact on an endangered primate species, the black howler monkey, Alouatta pigra. Four hundred and seventy-one visitors were surveyed to assess their travel goals, conservation commitments, and reactions to viewing howler monkeys. Data were also collected on the behaviour of tourists and monkeys during encounters. More intense tourist interactions with howler monkeys were correlated with the number of tourists and the duration of the encounter; guided parties interacted more intensely than unguided parties. Tourists were largely unaware that these interactions may harm the howler monkeys. Qualitative observations of howler response to tourists suggest short- and long-term negative impacts. These impacts could be mitigated through more effective guide training, limiting tourist group size, and increasing entrance fees at the Reserve. Improving environmental education may reduce impacts and motivate some tourists to become advocates for conservation of endangered species.