This article examines the sexual behaviour and the HIV/AIDS knowledge and vulnerability of female street hawkers in Lagos metropolis, Nigeria. A total of 126 female street hawkers under 18 were sampled in a cross-sectional survey and six Focus Group Discussions (FGDs) were conducted to generate data from respondents. Data on sexual behaviour reveal that 79.4% of respondents are sexually active, while 62.6% of them initiated sexual intercourse at 11 years old or below. Data on knowledge about HIV/AIDS show that all the respondents have heard about the pandemic. Specifically, 64.3% and 35.7% of respondents mentioned sexual intercourse and blood contact as major routes for HIV transmission, respectively. Only 7.1% of the respondents use condoms during sexual intercourse. FGD data show significant relationships between sexual behaviour and condom use and the poverty level of the families, parental neglect, and poor parent-child communication. Since female street hawkers in Nigeria are highly vulnerable to HIV infection, the government and NGOs working on the prevention of HIV/AIDS should pay special attention to children, especially urban street hawkers.