Although studies have been conducted in neighboring countries, there are no published data on men who have sex with men (MSM) in mainland Tanzania. We report on a respondent-driven sampling study of 271 MSM in Dar es Salaam, Tanzania. The sample covered a wide range of educational attainment and employment, median age was 24, and all respondents had heard of HIV/AIDS, mostly through public media. Those satisfied with media information on HIV were younger, had lower education, and had obtained their information from health facilities. Over two-thirds believed that having one faithful partner and using condoms would protect against HIV: nevertheless, more than two-thirds were worried about HIV infection. Two-thirds had had a relationship with a woman, one-third in the past year. Predictors of non-use of condoms for anal sex with last casual partner were younger age, not being worried about HIV infection, and agreeing to have sex even if a condom was refused. There was no significance in proportion using a condom with last casual (43%) and last regular (49%) partner. Most partners (MSM knew a median of 10 other MSM) were met in bars, music halls, and in the home/local environment, and 70% of MSM described their sexual position as "bottom." Sixty percent reported having an HIV test and the great majority was comfortable discussing condoms with partners and friends: half would refuse to have sex if condoms were not agreed to. These data suggest a significant "gay" community in Dar es Salaam with relatively accurate HIV information but moderate condom use, HIV testing and ability to refuse unsafe sex. There is clearly scope for targeted HIV prevention programs in the MSM community in Tanzania.