HIV/AIDS programmes and interventions are more likely to succeed if they engage with local people's beliefs about AIDS causation. This study examined beliefs about general illness, sexually-transmitted infection (STI) and AIDS aetiology in rural Mwanza, Tanzania. From 1999-2002, participant observation was carried out in nine villages for a total of 158 person-weeks. Beliefs about general illness causation included God's will, chance, natural/biological, ancestral spirits, and witchcraft. STIs were generally attributed to natural causes, but beliefs about AIDS causation were more complex. Few villagers had heard of HIV, but most had heard of AIDS and understood that AIDS could be contracted through sex. A small proportion of villagers knew that such an infected person might appear healthy, but they generally believed the asymptomatic period to last only a few months after exposure; if healthy beyond that, the person was not believed to have been infected. Many people in all villages reported belief in both a 'real' (natural) AIDS, which leads to certain death, and a similar illness caused by witchcraft, which can be cured using traditional medicine. Punishment of accused witches occurs officially and informally, and this may increase with increasing AIDS deaths. There is an urgent need for culturally appropriate interventions to address HIV/AIDS causation beliefs in the region.