PIP: The Center for African Family Studies (CAFS) together with the Family Planning Association of Tanzania (UMATI) investigated the determinants of male fertility and sexual behavior in the Mbeya and Rukwa regions of Tanzania. 1582 men aged 15-54 years were interviewed and 62 men participated in focus group discussions. Data were collected on respondents' sociodemographic characteristics; marital status and type of marriage; achieved fertility; fertility preferences and intentions; family planning knowledge, attitudes, and practices; cultural and traditional values and taboos; and knowledge of sexually transmitted diseases, including HIV. It was found that men in the Mbeya and Rukwa regions become sexually active very early in life, with about 27% of respondents beginning to live with a woman before age 20 years. 12.5% were in polygamous unions, procreation is the main reason people marry, and approximately 28% of men with nine or more children wanted additional children. Limited fertility differentials were observed by area of childhood residence, education, and religion. 32% preferred to have sons and old-age security remains a major motivating factor for having many children. Slightly more than 28% of men had extraconjugal relations with five or more different partners during the 6 months before the study. However, about 57% of these men did not use condoms.