One of the thorniest issues regarding language in education in the majority of post-colonial countries is the issue of the language(s) of instruction. This study examines how Moroccan high school science teachers perceive the language of instruction when teaching scientific subjects. The Frame Law 51 17, which stipulates the use of foreign languages in the teaching of scientific subjects, is the point of departure. As such, the study main objective is to explore the appropriate language(s) of instruction in regards to the teaching of scientific subjects from high school science teachers’ perspective. In this study, a questionnaire was administered to 66 high school science teachers randomly chosen. The questionnaires were analyzed by SPSS. In parallel, interviews were conducted with nine science teachers which were analyzed using Braun and Clarke’s 2006 Thematic Analysis. Different results were revealed most importantly the positive attitudes that science teachers have towards English as a means of instruction, the incompatibility of language planning and policy (LPP) with the teaching/learning process needs inter alia. These results can be seen of great importance for syllabus designers, researchers in the field and teachers to understand more the nature of the issue. Accordingly, some recommendations to better the teaching/learning process regarding scientific subjects are suggested.