Utilizing part of the survey data collected for a National Institute on Drug Abuse (NIDA)-funded project from 29 public elementary schools in Phoenix, Arizona (N = 1,600), this study explored the underlying structure of Mexican-heritage youths' ethnic identity and cultural/linguistic orientation. Latent profile and transition analyses identified four distinct orientation profiles endorsed by the early adolescents and their developmental trends across four time points. Most Mexican and Mexican American adolescents endorsed bicultural profiles with developmental trends characterized by widespread stasis and transitions toward greater ethnic identity exploration. Multinominal logistic regression analyses revealed associations between profile endorsement and adolescents' gender, socioeconomic status, parents' birthplace, and visits outside the United States. These findings are discussed in regard to previous findings on acculturation and ethnic identity development. Individuals' adaptation to the immediate local environment is noted as a possible cause of prevalent biculturalism. Limitations and future directions for the research on ethnic identity development and acculturation are also discussed.