Previous research has shown that the perception of speech sounds is strongly influenced by the internal structure of maternal language categories. Specifically, it has been shown that stimuli judged as good exemplars of a phonemic category are more difficult to discriminate from similar sounds than bad exemplars from equally similar sounds. This effect seems to be restricted to phonemes present in the maternal language, and is acquired in the first months of life. The present study investigates the malleability of speech acquisition by analysing the discrimination capacities for L2 phonemes in highly proficient Spanish-Catalan bilinguals born in monolingual families. In Experiment 1 subjects were required to give goodness of fit judgments to establish the best exemplars corresponding to three different vowel categories (Catalan /e/ and //; Spanish /e/). In Experiments 2 and 3, bilinguals were asked to perform a discrimination task with materials in their maternal language (Exp. 2) and in their second language (Exp. 3). Results reveal that bilinguals show a reduced discrimination capacity only for good exemplars of their maternal language, but not for good exemplars of their second language. The same pattern of results was obtained in Experiment 4, using a within-subjects design and a bias-free discrimination measure (d'). These findings support the hypothesis that phonemic categories are not only acquired early in life, but under some circumstances, the acquisition of new phonemic categories can be seriously compromised, in spite of early and extensive exposure to L2.