This study examined the capacity of 4-month-old infants to identify their maternal language when phonologically similar languages are contrasted, using a visual orientation procedure with a reaction time measure. Infants from monolingual and bilingual environments were compared in order to analyze whether differences in linguistic background affect this behavioral response. In experiment 1 the validity of the procedure was assessed with a pair of phonologically dissimilar languages (Catalan or Spanish vs. English). In experiment 2, 20 infants from monolingual environments tested in a similar language contrast (Catalan vs. Spanish) indicated that discrimination is already possible at that age. Results from experiment 3, using low-pass filtered utterances, suggested that infants can rely on information about supra-segmental features to make this distinction. For the infants growing up in bilingual environments no preference for either of the familiar languages was found. Moreover, when their maternal language was contrasted either with English or with Italian, in both cases the bilingual group showed a similar pattern, consisting of significantly longer latencies for the familiar language. Possible interpretations of this unexpected pattern of results are discussed and its implications for bilingual language acquisition are considered.