This paper explores sub-disciplinary variations and generic structure of research article introductions (RAIs) in three sub-disciplines of applied linguistics (AL), namely, English for Specific Purposes (ESP), Psycholinguistics, and Sociolinguistics, using Swales's CARS model. The corpus consisted of 90 RAIs drawn from a wide range of refereed journals in the corresponding sub-disciplines. The results indicate sub-disciplinary variations in the structure of this genre that can be justified through the concepts of 'established' versus 'emerging' fields. The findings underline the need for further in-depth research into sub-disciplinary variation and generic structure of RAIs. The results also have pedagogical implications for ESP/EAP practitioners and course designers as well as material developers.