While listening to non-native speech, second language users filter the auditory input through their native language. We examined how bilinguals perceived second language (L2 English) sound sequences that conflicted with native-language (L1 Spanish) constraints across three experiments with different task demands. We used the L1 Spanish phonotactic constraint (i.e., rule for combining speech sounds) that vowels must precede s+consonant clusters (e.g., Spanish: estricto, “strict”). This L1 Spanish constraint may influence Spanish-English bilinguals’ processing of L2 English words such as strict because of a missing initial vowel, as in estrict. We found that the extent to which bilinguals were influenced by the L1 during L2 processing depended on task demands. When metalinguistic awareness demands were low, as in the AX word discrimination task (Experiment 1), cross-linguistic effects were not observed. When metalinguistic awareness demands were high, as in the vowel detection (Experiment 2) and lexical decision (Experiment 3) tasks, response times demonstrated that bilinguals were influenced by the L1 constraint when processing L2 words beginning with an s+consonant. We conclude that bilinguals are cross-linguistically influenced by L1 phonotactic constraints during L2 processing when metalinguistic demands are higher, suggesting that L2 input may be mapped onto L1 sub-lexical representations during perception. These results extend previous research on language co-activation and speech perception by providing a more fine-grained understanding of task demands and elucidating when and where cross-linguistic phonotactic access is present during bilingual comprehension.