The relationship between reproductive maturation and the onset of sexual behavior in boys across cultures is unclear. To explore the relative timing of pubertal events and their relationship to the onset of sexual behavior, we used data collected from 442 Zimbabwe school boys aged 12-18 years. Measures of reproductive maturation included self-reported spontaneous nocturnal emission, secondary sexual characteristics, and salivary testosterone. Behavioral measures included age at first sexual fantasies, non-coital sexual behavior, and coitus. Sigmoid interpolation curve techniques indicated a median age of first erection at 10.75 +/- 0.11 years, first sexual fantasy at 12.66 +/- 0.03 years, first spontaneous nocturnal emission at 13.02 +/- 0.03 years, and adult levels of blood testosterone at 17.2 +/- 0.7 years. First reported spontaneous nocturnal emission was significantly related to all measures of sexual behavior. Multiple regression models indicated that first reported spontaneous nocturnal emission was a stronger predictor of sexual behavior than variation in secondary sexual characteristics. In addition, testosterone was a significant predictor of sexual fantasies and intercourse, independent of first reported spontaneous nocturnal emission. These results confirm earlier findings in other populations that self-reported spontaneous nocturnal emissions can be used as a marker of pubertal timing among adolescent boys. They also suggest that variation in testosterone plays a role in the onset of sexual behavior among boys, beyond its relationship to developmental timing. The behavioral mechanisms represented by the significant effects of testosterone reported here remain to be investigated.