Few sub-Saharan African countries have witnessed declines in HIV prevalence, and only Uganda has compelling evidence for a decline founded on sexual behavior change. We report a decline in HIV prevalence in eastern Zimbabwe between 1998 and 2003 associated with sexual behavior change in four distinct socioeconomic strata. HIV prevalence fell most steeply at young ages-by 23 and 49%, respectively, among men aged 17 to 29 years and women aged 15 to 24 years-and in more educated groups. Sexually experienced men and women reported reductions in casual sex of 49 and 22%, respectively, whereas recent cohorts reported delayed sexual debut. Selective AIDS-induced mortality contributed to the decline in HIV prevalence.