Informal movement of domesticated poultry and wild birds is considered a major threat in terms of highly pathogenic avian influenza H5N1 transmission between birds and from birds to humans. However, the risk of transmission from informal illegal poultry movement has received little attention in Indonesia where human fatalities are the highest in the world. This research investigated the illegal movement of adult poultry between the islands of Java, Bali and Lombok to determine the potential risk of HPAI H5N1 transmission. The aim was to determine known origins and destinations of poultry, estimated quantity and types of birds, people involved and the drivers of illegal movement. Transportation and handling methods and views on how to minimise illegal movement were also investigated. In-depth interviews were carried out with 71 key informants in Bali and Lombok in 2009. East Java was the main origin of poultry entering Bali, followed by Central Java and Lombok. Interviewees estimated that over 10,000 village chickens, 500 ducks and 50 fighting cocks were brought into Bali per month from all origins. However, there were significant discrepancies with quarantine records indicating that the majority of birds imported illegally are not detected. We conclude that although informal illegal movement of poultry in Indonesia poses a potentially high risk for potential HPAI H5N1 transmission if birds are infected, much can be done to increase surveillance, encourage reporting of sick birds, educate traders about the risks and provide effective quarantine within an appropriate cultural framework.