The goal of the study was to assess differences between native Greek and bilingual, immigrant children of Albanian descent learning Greek as a second language on a receptive vocabulary measure. Vocabulary measures were obtained at five time points, 6 months apart, from 580 children attending Grades 2–4. Individual variability on both initial performance (intercept) and growth rate (slope) was assessed using hierarchical linear modeling, which included linguistic/ethnic group, parental education (as a socioeconomic status [SES] indicator), gender, and a measure of nonverbal cognitive ability as time-invariant predictors of vocabulary growth. Results indicated that linguistic/ethnic group, parental education, and baseline nonverbal cognitive ability were significant predictors of initial vocabulary scores, whereas only linguistic/ethnic group and nonverbal ability accounted for significant variability in vocabulary growth rates. Additional analyses confirmed that linguistic/ethnic group remained a significant predictor of receptive vocabulary knowledge at both the intercept and the slope levels even after controlling for the initial differences between groups on parental education and block design subtest scores.