South African student teachers were studied to see how they coped with requirements to teach science using argumentation. Lesson observations, plans, reflective logs, post-teaching interviews and assessment of pupils' argumentation were used to compare student teachers' preparedness and interactions with pupils. Two clusters of students were identified representing high preparedness and low interaction. A high degree of preparedness alone did not guarantee high levels of argumentation. Schools' educational situations were independent of success in teaching argumentation. The outcomes and implications for further development of teaching critical thinking are discussed. (Contains 3 tables and 1 figure.)