Philosophiae Doctor - PhD / It has been noted by Lin (2013) that studies on multilingual talk, as illustrated by code switching in the classroom, have been repetitive and descriptive, and have for a while not been underpinned by substantially new or different questions (Lin, 2013:15). First, many of the studies in the literature have, for instance, concluded that there is a functional allocation of languages (FAL) in multilingual classroom teacher talk (e.g. Baker, 2012; Martin, 1996; Probyn, 2006, 2014; Jegede, 2012; Modupeola, 2013; Salami, 2008), such that language „a‟ is used for presentational knowledge, and language „b‟ is used for explanatory knowledge, and these claims have not been subjected to sustained scrutiny. Secondly, codeswitching and translanguaging increasingly have been the dominant and exclusive frameworks used, and this has limited the kinds of insights that can be obtained or the kinds of questions that can be posed. Thirdly, where the effects of multilingual teacher talk on students‟ understanding or knowledge are at all captured in studies, such effects have either been based on researcher intuition or have not been the object of sustained empirical demonstration. Fourthly, many studies have assumed merely that it is the configuration of languages that produces claimed effects of multilingual teacher talk, and attention has hardly been paid to repetition of content or to knowledge structure. Fifthly, it is not often the case that studies or findings are presented in a nuanced form that takes into account the possible effect of different subject types, school types or levels of study. Sixthly, and overall, many studies making claims on the effect of teacher‟s code-switching or trans-languaging on students‟ knowledge do not theoretically engage with knowledge, beyond the distinction between presentational and explanatory forms of knowledge, thus illustrating what Maton (2013) regards as “knowledge-blindness” (that is, the paradox of limited engagement with knowledge structures in pedagogical research making knowledge claims). As a result, little is known about how specific units of knowledge are encoded according to categories in a theory of knowledge, how knowledge encodings interface with languages, and how composite knowledge structures-language profiles can be visualised. This study draws on Legitimation Code Theory Semantic and Terminology Theory in order to investigate the interface of language and knowledge in multilingual teacher-talk in science and business studies classrooms in Yola, North-Eastern Nigeria. This focus should make it possible to answer questions such as the following which, though important, have not often been posed on account of the limited engagement in the research on classroom multilingualism with theories of knowledge: a) to what extent is it appropriate to claim that there is a functional allocation of language in multilingual teacher-talk (in which language „a‟ is used for so-called presentational knowledge, and language „b‟ for explanatory knowledge)?; b) what kinds of encodings of knowledge occur in a set of science and business studies lessons?; c) given documented visual patterns of knowledge dynamics emerging from recent research in the sociology of knowledge (e.g. semantic waves, semantic flatlines both high and low, downward shift and upward shift), (Maton: 2013, 2014a, 2014b), what knowledge profiles are observable and how does language use in multilingual teacher-talk map onto these patterns?; d) how are any observed differences in the composite knowledge-language profiles to be explained?; and e) what effects do various language-knowledge profiles have on students‟ understanding of the lesson and on their demonstration of their knowledge? Data for the study was derived from transcripts of audio-recorded multilingual teacher-talk in two subjects (integrated science and business studies) as taught in grades seven and nine in four secondary schools (two private and two public schools) in Yola, North-East Nigeria. Findings show, among others, that it is not always the case that the official classroom language (English) is used for introductory discourses, and the non-official classroom languages are used for explanatory discourses. Findings further reveal that it is not primarily the functional allocation of languages that explains perceptions or empirical claims of enhanced student understanding. We also observed that the number of content iterations, combined with knowledge structures, is an important factor that enhances or explains the performance of students. While this research has paid a lot of attention to teacher talk in the classrooms in two sites in Yola, North-East, Nigeria, where the use of Hausa and Fulfulde languages by the students is mainly in the spoken form, it would be interesting for future research to replicate this type of study in an environment where the non-official language of the classroom is perhaps used more frequently in reading and writing.