The World Health Organization (WHO) has identified a failure to provide education for vulnerable populations such as prisoners as a contributing factor to the epidemic of sexually transmissible infections (STIs). Despite this recognition, little is known about prisoners' level of knowledge of STIs compared with the general population. Using computer-assisted telephone interviews, we compared a representative sample of 2289 Australian prisoners, aged 18-59 years from New South Wales and Queensland prisons with a representative community sample of 3536 participants from these two states. Prisoners had significantly better knowledge than the general community of chlamydia-related questions, while knowledge of herpes (genital and oral) was slightly better in the community sample. Prisoners who were aged over 25 years, not married, female, self-identified as either homosexual or bisexual and reported a history of STIs tended to have better STI knowledge levels. Despite their more disadvantaged backgrounds, prisoners demonstrated relatively good health literacy in relation to STIs. Ongoing education about the transmission risks of STIs for prisoners and the general community is needed.