The effect of negation training in a second language on the expression of negation in the native language was investigated. Four-year-old children from bilingual (Spanish/English) homes who showed no expressive or receptive ability in Spanish negation and were either proficient or nonproficient in English negation received Spanish negation training. Children who were proficient in English negation maintained correct responses in English and showed increased correct responses in Spanish following simultaneous training in both languages or in Spanish alone. Children who were nonproficient in English negation demonstrated a decrease in correct English responses following training in Spanish alone; however, children who received training in English and Spanish simultaneously showed increases in correct responses in both languages. These findings suggest that language training programs with children learning a second language should consider the relationship of the two language training conditions (simultaneous vs. independent) with the child's level of native language proficiency.