In pursuance of one of the goals of the new South African curriculum to integrate science with Indigenous Knowledge Systems (IKS), this study, which forms part of a larger study, is concerned with determining teachers' and learners' conceptions of a number of familiar natural phenomena. Several recent studies have indicated that: (1) learners do hold cultural beliefs which hinder them from learning the scientific way of interpreting natural phenomena; (2) argumentation is an effective instructional tool that teachers can use to enhance learners' understanding of such phenomena; and (3) argumentation helps to create necessary opportunities for learners to externalize their thoughts, clear their doubts or even change their minds about what they learn inside or outside the classroom. Specifically, this study used an Argumentation-Based Course (A-B Course), a pre-post-test questionnaire and structured individual interviews to explore conceptions of a number of selected phenomena (including the rainbow) that were held by a group of nine experienced science teachers. The findings show that the teachers' apparent scientific conceptions of the rainbow were in reality embedded within an amalgamation of scientific and IKS-based cosmological worldviews. The A-B course seemed to have made the teachers to become more favourably disposed to the teaching of IKS along school science. They were also enthusiastic about higher institutions including such a course in their teacher training programmes.