The importance of argumentation in science education is outlined and the relatively low level of argumentation typically observed in classrooms in the UK is noted, along with possible reasons for this. The research sets out to determine the extent to which primary school pupils engage in argumentation and to characterise their arguments in primary science lessons. A provisional framework is developed for analysing argumentation in this setting. Transcripts of pupils arguing are used to illustrate how pupils co-construct arguments without teacher intervention or guidance. A number of factors which appear to influence argumentation are noted.