In this paper, we analyse long-term whale shark Rhincodon typus sightings collected by ecotourist operators and evaluate the validity of conclusions drawn from the data for scientific and conservation purposes. To date information about the basic ecology and movements of whale sharks is sparse, and only recently has the species received global conservation attention. A dive company in Phuket, Thailand, documented whale shark sightings in the Andaman Sea for 10 years along 300 km of coastline. Whale shark sightings, corrected for effort, dropped by 96% between 1998 and 2001. Combining the seasons from 1992 to 1998, the number of whale shark sightings increased significantly from October to May. The sizes of sharks observed suggest that the majority were juveniles. We discuss the limitations of using ecotourist operators as non-specialist volunteers for data collection but conclude that their use can be beneficial for long-term, broad geographic studies such as this.