A simulation model was used to study the spread of two sexually transmitted diseases (STDs), namely gonorrhea and genital infection with Chlamydia trachomatis. The model is based on a stochastic pair formation and separation process, which describes the underlying structure of the sexual contact pattern. It is implemented as a Monte Carlo simulation model. Spread of the STDs was modeled in an age-structured heterosexual population with a highly sexually active core group. Contact tracing strategies, screening of various subgroups, and the effect of condom use were compared. The authors conclude that contact tracing is very effective as a prevention strategy, that screening should be targeted to the highly active core group, that age is not sufficient as a determinant for high sexual activity to make screening of certain age groups useful, and, finally, that consistent condom use by a fraction of the population can contribute substantially to the prevention of STDs. All strategies proved more effective for gonorrhea than for Chlamydia prevention, which may explain the relatively high prevalence of Chlamydia found in many heterosexual populations.