Many sea-birds are dependent for breeding on islands free from both mammalian predators and high levels of human disturbance. Yet human use of small islands appears to be increasing in many parts of the world, including the >150 islands in the Gulf of California, México. For this reason, the frequency and activities of human visitors to San Pedro Mártir Island, one of the most important sea-bird nesting sites in México, were studied over the course of 14 months of field work between 1990 and 1992. On average >350 people per month visited the near-shore waters. The most important visitors were: (1) commercial fishers in small open boats called pangas; (2) commercial fishers in larger ships; (3) private sport-fishers; and (4) commercial environmental-tourism groups. Private sport-fishing boats were most frequently observed, but private sport-fishers rarely landed on shore and caused little apparent disturbance to nesting or roosting sea-birds or California sea-lions (Zalophus californicus). Commercial companies concerned with environmental tourism also caused little apparent disturbance to sea-birds or sea-lions, but only because the company that brought the majority of 'ecotourists' developed and followed guidelines to minimize disturbance. Disturbance caused by all tourists can be reduced at minimal cost to both tourists and protected-area managers. Commercial fishers were responsible for most of the apparent disturbances to sea-birds and sea-lions; decreasing this apparent impact will be more difficult. Most apparent human impact was found to be short term and localized. However, commercial fishers and scientific researchers have the potential to cause long-term changes to the terrestrial ecology of the island by introducing exotic species.