The research reported in this paper investigates the impact of, and challenges with, encouraging argumentation on students' conceptual development in physics. It draws its theoretical framework from two socio-cultural perspectives of learning, i.e. that learning science occurs in a social setting and secondly that learning science is a discursive and cultural process. A three stage teaching sequence intended to encourage argumentation was implemented at a government high school in Maseru in 2009. Data were collected through audio taping of group discussions and students' written reports. The results show that the introduction of argumentation as a strategy of introducing learner-centred approaches in science classrooms proved to be beneficial for concept development. The challenges of practicing argumentation include the use of second language and enculturation of students into a scientific community. Students carried out part of their discussions in Sesotho the first language, instead of English the second language which is used as the medium of instruction. Students also used short sentences that needed elaboration and rarely questioned one another's justifications.