This study investigates the reporting of premarital sex in rural southern Malawi. It summarizes the results of an interview-mode experiment conducted with unmarried young women aged 15-21 in which respondents were randomly assigned to either an audio computer-assisted self-interview (ACASI) or a conventional face-to-face (FTF) interview. In addition, biomarkers were collected for HIV and three STIs: gonorrhea, chlamydia, and trichomoniasis. Prior to collecting the biomarkers, nurses conducted a short face-to-face interview in which they repeated questions about sexual behavior. The study builds on earlier research among adolescents in Kenya where we first investigated the feasibility and effectiveness of ACASI. In both Malawi and Kenya, the mode of interviewing and questions about types of sexual partners affect the reporting of sexual activity. Yet the results are not always in accordance with expectations. Reporting for "ever had sex" and "sex with a boyfriend" is higher in the FTF mode. When we ask about other partners as well as multiple lifetime partners, however, the reporting is consistently higher with ACASI, in many cases significantly so. The FTF mode produced more consistent reporting of sexual activity between the main interview and a subsequent interview. The association between infection status and reporting of sexual behavior is stronger in the FTF mode, although in both modes a number of young women who denied ever having sex test positive for STIs/HIV.