This study examined lexical organization in English language learners (ELLs) who acquired their second language (L2) either during or after the period of maximal sensitivity related to the critical period hypothesis. Twenty-three native-Spanish-speaking ELLs completed psycholinguistic tasks to examine age effects in bilingual lexical organization. The tasks involved the use of primes, or stimuli presented for very short time frames (below a level of conscious perception), followed by word--nonword lexical decision tasks with translation and semantic relationships to the primes. Primes presented in the first language facilitated speed of lexical decision in L2, but not the reverse. Reaction times were faster for translation than for semantic primes, with no differences related to age of acquisition or prime duration. However, participants who acquired L2 later showed different error patterns than those who acquired L2 earlier. Results are discussed with regard to the critical period hypothesis in second language acquisition.