Animal parks and exhibits which offer visitors the option of close interaction with the animals displayed have been very successful with some species, including primates. However, there is always an element of risk to the visitor who enters another species' environment and interacts with its members. This risk factor may cause particular concern when the exhibit animal is a nonhuman primate because of the transmission of various zoonoses. The Barbary macaques (Macaca sylvanus) at Queen's Gate, Gibraltar, attract large numbers of tourists who are permitted to feed them. This animal-human contact sometimes results in monkeys biting visitors. Bite frequency data from hospital records for 1980–1989 and direct observations (July 1990–June 1991) form the basis of this study. Biting is a density-dependent phenomenon since monkey-visitor aggression rates increase with visitor numbers. Time of day affects bite rates, more people being bitten in the afternoon—the main visitation time. Likewise, more bites are recorded during summer months when visitor numbers are higher. A tendency towards acclimatization of the monkeys to the very large number of visitors appears after 1985. Rank correlation coefficients between visitor numbers and bites per month fall from 1985 to 1989, reflecting a drop in visitor density response. Analysis of visitor profiles indicates a negative correlation between visitor age and likelihood of being bitten. However, women are bitten significantly more than men. Average age of people bitten also varies between months. Recorded aggression levels are generally low despite no control over animal-visitor interactions, but are much higher than in similar Barbary macaque enclosures in France and Germany. The Gibraltar troop's small home range and high visitor density may explain recorded bite levels. Effective visitor education can significantly lower biting incidence.