urrent evidence suggests that anal intercourse (AI) during sex work is common in sub-Saharan Africa, but few studies investigated the contribution of heterosexual AI to HIV epidemics. Using a respondent-driven sampling survey of female sex workers (FSW) in Abidjan (2014), we estimated AI prevalence and frequency. Poisson regressions were used to identify AI determinants. About 20% of FSW (N = 466) engaged in AI during a normal week (95% confidence intervals: 15-26%). Women who performed AI were generally younger, had been selling sex for longer, were born in Côte d'Ivoire, reported higher sex-work income, more frequent sex in public places, and violence from clients than women not reporting AI. Condom use was lower, condom breakage/slippage more frequent, and use of water-based lubricants was less frequently reported for AI than for vaginal intercourse. Using a dynamic transmission model, we estimated that 22% (95% credible intervals: 11-37%) of new HIV infections could have been averted among FSW during 2000-2015 if AI had been substituted for vaginal intercourse acts. Despite representing a small fraction of all sex acts, AI is an underestimated source of HIV transmission. Increasing availability/uptake of condoms, lubricants, and pre-exposure prophylaxis for women engaging in AI could help mitigate HIV risk.