Social organization and structure can often be inferred through language use in most languages. Japanese, in particular, has a linguistic system which overtly marks and encodes social structure, behaviour and patterns of affiliation. This set of complex sociolinguistic rules is internalized and strictly observed by native speakers and deviation from established is often not welcomed. This paper studies the level of control over these sociolinguistic rules which non-native speakers of Japanese possess when interpreting consecutively from English to Japanese. The study involves seven Australian student interpreters. An analysis of the students’ performance indicates that despite having a decent level of linguistic competence in Japanese, the students had difficulty in using the appropriate speech levels required in Japanese, resulting in ‘unnatural language’. The types of errors are categorized and implications are drawn for the teaching of Japanese. The significance of the findings to wider interpretation and translation issues is also discussed.