This study examines possible semantic interaction in fully fluent adult simultaneous and early second language (L2) bilinguals. Monolingual and bilingual speakers of Spanish and English (n = 144) were tested for their understanding of lexical categories that differed in their two languages. Simultaneous bilinguals came from homes in which Spanish or Spanish and English were spoken when they were children, and L2 bilinguals entered the US as children. Accuracy data show higher ultimate attainment of language-specific semantic knowledge in English than in Spanish, but in both languages the interaction of the semantic categories with conceptual knowledge is observable. The data reveal subtle differences in early bilinguals’ extensions of words, but only in some types of categories, and modified by level of proficiency.