Several lines of evidence suggest the importance of phonological working memory (PWM) in language acquisition. We investigated the neural correlates of PWM in young adults who were under compelling social pressure to be bilingual. Equal bilinguals had high proficiency in English and Chinese as measured by a standardized examination, whereas unequal bilinguals were proficient in English but not Chinese. Both groups were matched on several measures of nonverbal intelligence and working memory. In-scanner behavioral results did not show between-group differences. Of the regions showing load-dependent increments in activation, the left insula showed greater activation in equal bilinguals. Unequal bilinguals showed greater task-related deac-tivation in the anterior medial frontal region and greater anterior cingulate activation. Although unequal bilinguals kept apace with equal bilinguals in the simple PWM task, the differential cortical activations suggest that more optimal engagement of PWM in the latter may correlate with better second-language attainment. phonological working memory ͉ functional MRI ͉ intergroup differences I n an increasingly global world, facility with two or more languages is a capability that confers competitive advantages. Of specific interest to the current investigation is why, despite being immersed in the same environment, some individuals have difficulty acquiring a second language even though they have an excellent command of their first language. § Prior neuroimaging studies have characterized the effects of age of acquisition (1, 2) and amount of linguistic experience in bilinguals (3) on brain activation but, to our knowledge, none have examined functional anatomy underlying differences in second-language acquisition ability. Several lines of evidence suggest that phonological working memory ¶ (PWM) plays a crucial role in determining language acquisition ability (4). It has been proposed that the phonological loop, consisting of a short-term phonological store and a sub-vocal rehearsal system, exists to facilitate language acquisition (5, 6). Specifically, the phonological loop is thought to temporarily store unfamiliar sound patterns, whereas more permanent representations are being constructed for long-term memory storage (4). In support of these postulates, a number of behavioral studies have shown that measures of PWM, such as digit span and word and nonword repetition, predict the outcome of native language acquisition in children (6-8) and foreign language acquisition in both children (9) and adults (10, 11). It has also been observed that polyglots have a larger PWM capacity than nonpolyglots (12). Further, neuropsychological studies of patients with defective short-term memory (13) and children with specific language impairment and low achievement (14, 15) provide additional support for the hypothesis that the PWM plays a crucial role in language acquisition. Taken together, these findings point to a compelling link between PWM and language acquisition and suggest that PWM is important for successful language acquisition. In this study, we investigated how the neural correlates of PWM might differ in young adults who achieved excellent grades in English but who differed in attainment as regards their second language (Chinese). Language proficiency was indexed by scores in standardized language examinations. These evaluated both oral and written language skills. To reduce the confounding effects of factors that could influence second-language acquisition , volunteers were matched for educational environment, scholastic performance, and performance in a number of standardized neuropsychological tests of nonverbal and verbal skills. To evaluate the neural correlates of PWM, participants were scanned while performing an auditory n-back test. Successful performance of this task requires continuous updating and temporal reordering of phonological information. We used stimuli that were phonologically unfamiliar to the volunteers to ensure that task performance would be minimally facilitated by the use of lexical and sublexical information. Item load was varied to reveal PWM load-dependent effects on activation. We expected areas engaged by PWM to show a monotonic increase in activation (16, 17). Previous theoretical and behavioral work points to the existence of two dissociable components in PWM: a subvocal rehearsal system and a short-term phonological store. Functional imaging results suggest that the subvocal rehearsal system is located in Broca's area 44 (BA 44), left premotor cortex (BA 6), and supplementary motor area (BA 6), whereas the phonological store resides in the left inferior parietal cortex (BA 40) (17, 18). The obligatory role of the frontal opercular areas in phonological processing has been recently demonstrated by using repetitive transcranial magnetic stimulation (19). We predicted that there would be group-level differences in activation in brain regions involved in PWM and that this difference would be more prominent with increasing PWM load. We made no specific prediction concerning the direction of the effect as previous studies have shown both increases (20) and decreases (21, 22) in brain activation after a linguistic task is learned. Methods Participants. Thirty neurologically normal, right-handed English-Chinese bilinguals participated in this study after giving informed consent. They were selected on the basis of responses This paper was submitted directly (Track II) to the PNAS office. Freely available online through the PNAS open access option.