Using a nationally representative sample of Guatemalan women aged 15 to 49 years, this study examines the ethnic differences in the likelihood of pre-union sexual initiation, pregnancy, and childbearing and whether assimilation exists, altering the level of such likelihood. Consistent with the argument that sexuality and reproduction are more decoupled from union formation among Ladinos than among the indigenous people, the results of the multilevel discrete-time hazard models demonstrate that Ladina women are significantly more likely to experience both sexual debut and first childbirth before union entry than indigenous women. However, the low likelihood of indigenous women is limited to those who reside in predominantly indigenous communities, and their residence in predominantly Ladino communities significantly increases their likelihood of experiencing both events. On the other hand, Ladinas' likelihood is unaffected by the ethnic composition of the communities. These findings suggest that the assimilation process in Guatemala extends to sexual and reproductive behavior and is uni-directional from indigenous to Ladino. Furthermore, the likelihood of both pre-union sexual initiation and childbearing among indigenous women in Ladino communities is even higher than those of Ladinas in both the indigenous and Ladino communities. This invites an inference that these indigenous women's sexual partners may be Ladino males and that their relationships are characterized by the ethnic hierarchy in Guatemala, in which indigenous people are assigned an inferior ethnic status.