The sociolinguistic phenomenon of bidialectism can significantly influence foreign-language learning. This study provides empirical evidence (drawn from the Greek Cypriot bidialectal community) for this influence and it supports the recommendation that foreign-language educators be trained in language-variation issues. The study's methodological basis consisted of observations of lessons in which English is taught as a foreign language and of students' written tests in the Passive Voice (PV). The findings revealed that, despite the policy that the standard variety (Standard Modern Greek) should be the only first-language variety used, the nonstandard variety (the Greek Cypriot dialect) is also commonly used alongside English by teachers. This usage was adopted by students who then tended to form the PV erroneously because they systematically avoided grammatical aspects (such as the Perfect aspect) which do not occur in the grammar of their nonstandard variety. The study contributes to the emerging realisation that first-language bidialectal issues need to be addressed in foreign-language classes.