Full text can be accessed at http://std.sagepub.com/content/14/9/596.shortIn this paper we attempt to identify factors that determine knowledge on prevention of sexually transmitted diseases (STDs) among primary school pupils in Tanzania using data extracted from the Family Life Education KAP Survey conducted in 1999. The data used consist of a sample of 18,564 pupils (about 50% males and 50% females) from all the 20 regions of Tanzania mainland. There were significant differences between boys and girls as regards knowledge of family life education in general. Boys tended to be more knowledgeable than girls about many aspects of family life education. The findings show that only 40.1% of the respondents were able to identify all the three STDs given in the questionnaire namely gonorrhoea, syphilis and AIDS. As regards modes of HIV transmission only 37.5% had full knowledge of these, while 48.8% knew condom use and abstinence as methods of STD prevention. The bivariate analysis showed significant relationships between knowledge of condom use and abstinence as methods of STD prevention with knowledge of STDs, HIV transmission, education level of the respondents, residence and parents' education for both boys and girls. However, knowledge of STDs, HIV transmission and residence came out clearly in the logistic regression as predictors of knowledge of condom use as a method of STD prevention for both sexes. For males, mother's education was also a significant predictor of knowledge of condom use as a method of STD prevention. Furthermore, a significant predictor of knowledge of condom use for female respondents was found to be the respondent's educational level. On the other hand, predictors of knowledge of abstinence as a method of STD prevention were knowledge of STDs and HIV transmission.