Bilingualism (Sanz, 2000), motivation (Pintrich, 1989), and language aptitude (Grigorenko, Sternberg, and Ehrman, 2000) are crucial individual differences that contribute to successful adult language learning. Since Gardner's (1985) seminal work on motivation, many studies have shown that motivation is dynamic and that it affects language development in many ways (Dornyei, 2005). We also know that bilingualism has clear benefits for successful third language (L3) acquisition (Bialystok, 2006, 2001; Da Fortuna & Siegle, 1995; Swain et al., 1990). Yet still at issue is how much bilingualism contributes to the success of L3 acquisition when taking into consideration other individual differences, such as motivation and aptitude. This study addresses the profiles of second language (L2) and third language (L3) learners of English, using 128 participants from the "Casa de Cultura Britanica's" English language program, which is affiliated with the "Universidade Federal do Ceara" in Fortaleza, Brazil. The participants completed the Portuguese version of the Motivated Strategies for Learning Questionnaire (MSLQ), "Questionario de Estrategias Motivacionais para Aprendizagem" (QEMA) (Brown et al., 2001), the Cognitive Ability for Novelty in Acquisition of Language (Foreign) (CANAL-FT) (Grigorenko et al. 2000), the Michigan State University English Language Test (MSUELT) (Cook, 2001), and a self-rated proficiency scale based on the Common European Framework of Reference (CEFR) (Hasselgreen, 2005). Analyses of the aforementioned factors, including ANOVAs, correlations, factor analyses, and discriminant function analyses compare the bilingual and multilingual participants, illustrating the differences between the groups. Motivational profiles, language aptitude scores, performance on the independent measure of English proficiency, and retention rates of the bilingual and multilingual participants are discussed using the aforementioned analyses. There is also a content analysis of interview data collected from a subgroup of the multilingual participants. One important finding of this dissertation includes the statistically significant results for the language aptitude scores between the bilingual and multilingual participants. This dissertation also supports the slowly growing body of research indicating that even a small amount of previous language learning experience can affect the process of learning subsequent languages. In addition, language learners who positively perceive interactions between languages studied might have advantages in subsequent language learning experiences. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]