The internationalization of Russian higher education contributes to the steady increase in the number of courses and programmes in the intermediary language, which in most cases is English, steadily gaining the status of the language of instruction. The article deals with the linguistic phenomena of code-switching and code-mixing in the context of university teaching of special disciplines in English, which is a foreign or a second language for all participants in the educational process - teachers and students. Particular attention is paid to the distinction between the terms “code-switching” (PC) and “code-mixing” (SC), which is of practical significance for the formation of the methodology of teaching special disciplines in an intermediate language as a special field of professional linguodidactics, as well as for the formation of the content of professional development programmes for university teachers. The phenomenon of code switching discussed in the article deserves close attention of linguists and psycholinguists, but in this article the author limits herself to studying speech behaviour of bilingual university teachers from the linguodidactic point of view. Thus, this study lies in the field of pedagogy and is an attempt to determine the nature of switching from English as the language of instruction or intermediary language to Russian as the native language in the aspect of the English-Russian code mixing and code switching. The subject under study in this case is the attitude of Russian-speaking university teachers to the role of the native language in the process of teaching the subject in English by means of a survey. The results of the survey, though subjective, allow us to make a primary statement that the observed transitions from the language of instruction (English) to the native language (Russian) in the teacher‟s speech represent cases of marked and conscious code switching, carried out in the context of the educational process, rather than code mixing. The study was conducted at St. Petersburg State University, the participants included teachers of special disciplines of a wide range of the humanities and sciences.