Analyses of individual-level risk factors have not been able to adequately explain why HIV has spread so extensively in southern Africa and why this has occurred especially within certain racial or ethnic groups. Using data from a longitudinal study of a representative sample of adolescents aged 14-22 living in Cape Town, South Africa, this article presents evidence of how differences in individual-level risk factors as well as sexual network structures between different racial or ethnic groups may help explain the differential spread of HIV in South Africa. Particular emphasis is placed on how levels of partner concurrency, respondent concurrency, mutual concurrency, serial concurrency and numbers of sexual partners and an average early age of sexual debut combine in different ways in the different racial or ethnic groups to create networks of sexual partnerships that differ in the density of their interconnections and hence potential for HIV spread. These network-level differences offer a potential explanation for the observed generalised HIV epidemic seen among the population of black South Africans.