A large body of research has documented how personality develops across adulthood, yet very little longitudinal work has examined whether these findings generalize beyond predominantly middle-class, highly educated White American or Western European individuals. This preregistered study uses longitudinal data from 1,110 Mexican-origin adults who completed a well-validated personality measure, the Big Five Inventory, up to six times across 12 years. Individuals generally maintained their rank ordering on the Big Five over time (rs = .66-.80), and the relative ordering of the Big Five within persons was also highly stable (rs = .58-.66). All of the Big Five traits showed small, linear mean-level decreases across adulthood. These trajectories showed few associations with sociodemographic factors (sex, education level, and IQ) and cultural factors (generational status, age at immigration, Spanish/English language preference, Mexican cultural values, American cultural values, and ethnic discrimination). The statistically significant findings we did observe mostly concerned associations between cultural values and Extraversion, Agreeableness, and Openness. Acquiescence bias was also positively associated with Big Five personality trait scores at every wave. There was no evidence of mean-level change in the Big Five when including time-varying acquiescence scores as covariates in the models. Divergences between the present findings and previous research highlight the need to study personality development with diverse aging samples. (PsycInfo Database Record (c) 2022 APA, all rights reserved).