The authors investigate the impact of the migrant labor system on heterosexual relationships on South African mines and assess the implications for the future transmission of human immunodeficiency virus (HIV) infection. The migrant labor system has created a market for prostitution in mining towns and geographic networks of relationships within and between urban and rural communities. A section of the migrant workforce and a group of women dependent on prostitution for economic support appear especially vulnerable to contracting HIV infection since they are involved in multiple sexual encounters with different, changing partners, usually without condom protection. Furthermore, sexually transmitted disease morbidity is extensive in the general and mineworker populations. Historically, migration facilitated the transmission of sexually transmitted diseases and may act similarly for HIV. Problems of combating the HIV epidemic in South Africa are discussed.