Many factors that contribute towards the perception of second language (L2) sounds and the subsequent establishment of L2 phonetic categories have been the focus of past research. However, learning a third language, unlike SLA, may be influenced by additional factors attributed to the presence of two language systems in a bilingual instead of one language system in a monolingual. Previous third language acquisition research, mainly lexical processing or syntax acquisition, have suggested that being a bilingual provides a positive influence to learners of a third language in attaining general proficiency. Whether bilingualism or a having multilingual benefit as a factor influences the acquisition of novel non-native speech contrasts by adult bilinguals, remains to be explored. Another factor that may facilitate in the phonetic acquisition of a third language is the role that phonetic features may play in the development of new phonetic categories. Features utilized in native contrasts may generalize in the learning of novel non-native contrasts, even if they play a limited or no role in the initial perception of these non-native contrasts. Such factors influencing third language acquisition within specific areas of proficiency like phonological acquisition have not been previously investigated. This study involves a perceptual training experiment examining the influence of factors of multilingual benefit and/or feature generalization. The training incorporates the acquisition of four speech contrasts (dental/alveolar-retroflex distinction) of a third language by bilingual (Bengali-English and Spanish-English speakers) and monolingual (American English speakers) groups. The perceptual data is gathered through measures like consonant identification, AX discrimination and perceptual assimilation. The perceptual assimilation data is analyzed using the PAM model to determine the extent to which the initial assimilation patterns change towards direction of learning following the training. The results across the testing measures show subtle effects of the multilingual benefit factor playing a role in facilitating the acquisition of the non-native speech contrasts when comparing individual language groups. This effect is observed as robust when both bilingual groups are combined to increase sample size during reanalysis in all tests, except for AX discrimination. The factor of feature generalization does not show an influence on the acquisition of non-native speech contrasts by Bengali-English bilinguals in any of the testing measures. Therefore, the results are suggestive of only the multilingual benefit factor playing a role in enhancing the acquisition process of a third language by adult bilinguals. The perceptual assimilation analysis across language groups show a change of assimilation types after training revealing a shift towards the direction of learning. The bilinguals show a greater shift from SC assimilation type to TC assimilation types than monolinguals. The study thus suggests that multilingualism as a factor facilitates the acquisition of non-native speech contrasts. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]