Online discussion forums are a popular learning tool commonly used in blended learning. Despite the widespread use of this tool, pre-service English as a Foreign Language (EFL) teachers’ conceptions of learning disciplinary concepts through online-discussion forums are under researched. This study aims at filling this gap by describing the conceptions of learning through such forums held by a group of 16 Mexican, pre-service EFL teachers enrolled in an undergraduate Second Language Acquisition course. Interviews were conducted and analysed using phenomenography. We found three experiential categories: Going over the content, Expanding knowledge metacognitively, and Negotiating knowledge. These involve different ways of approaching course material, writing and responding to messages, and relating to the teacher. WhatsApp was used as a parallel forum to negotiate potential conflicts without the instructor’s presence. The results suggest the need to train pre-service teachers to use online discussion forums (ODFs) with a deep approach to learning that includes meaning negotiation.