We examined the association between proficiency in instructed second language acquisition (ISLA) and previous bilingualism, starting age of ISLA, language anxiety and attitude. The analyses were conducted on 564 adolescent Australian twins. Additionally, by examining discrepancies within approximately 100 pairs of monozygotic twins, we sought to specifically identify the environmental effects related to attitude and anxiety on achievement (i.e. with genetic effects removed). We found a clear relationship between attitude towards language learning and proficiency in the second language. Furthermore, the analyses on the monozygotic twins point to the possibility that higher language anxiety is associated with higher proficiency. On the other hand, bilingualism and starting age of ISLA appear to be unrelated to proficiency in the language being learned.