The objective of this paper is to describe the sexual behaviour of 665 unmarried men aged 15-24, and explore factors associated with condom use at last intercourse. We use data from a cross-sectional survey conducted in economically marginal areas of Phnom Penh and Kratie town, Cambodia. One-third of respondents reported intercourse and half had had sex by the time they were 23. Of these, 39% had given money or gifts in exchange for sex. Transactional sex often occurred in the company of other males, and condom use was higher among those males compared with those who were alone. Of all sexually active respondents, half reported three or more partners, and 71% used a condom at last sex. Regression results showed that condom use varied by type of partner, was less likely among males outside the education system, and higher among those more positive and informed about condoms. Our findings highlight the need for HIV prevention efforts to encourage young men to use condoms with all intimate partners, promote advantages of condoms for both disease and pregnancy prevention, and address the needs of young men no longer in education. Further research, to understand the positive influence that peers have on condom use in certain circumstances, would be useful.