Entertainment-education is the process of designing and implementing an entertainment program to increase audience members' knowledge about a social issue, create more favorable attitudes, and change their overt behaviors regarding the social issue. The results of a field experiment in Tanzania to measure the effects of a long-running entertainment-education radio soap opera, Twende na Wakati (Let's Go with the Times), on knowledge, attitudes, and adoption of human immunodeficiency virus (HIV)/acquired immune deficiency syndrome (AIDS) prevention behaviors are presented. Multiple independent measures of effects and the experimental design of this study confer strong internal and external validity regarding the results of this investigation. The effects of the radio program in Tanzania include (1) a reduction in the number of sexual partners by both men and women, and (2) increased condom adoption. The radio soap opera influenced these behavioral variables through certain intervening variables, including (1) self-perception of risk of contracting HIV/AIDS, (2) self-efficacy with respect to preventing HIV/AIDS, (3) interpersonal communication about HIV/AIDS, and (4) identification with, and role modeling of, the primary characters in the radio soap opera.