This article reports the findings of a research project concerned with children engaging in scientific argumentation. Discussion activities were designed to enable groups of children to use evidence when making decisions. The findings show a variation in the success with which children construct scientific arguments; some groups debate most of the evidence and consider a range of options, whilst others explore a limited range of options and discuss only the evidence that supports their choice. Analysis of the data indicates that the roles children adopt have an important influence on how evidence is used. Drawing of the work of Belbin, a management theorist who explored the roles adopted by members of successful teams, key roles emerged and a taxonomy of children's roles was devised. This article explains the characteristics of these roles and their effect on the way the groups use evidence when making decisions in science. (Contains 6 tables, 1 figure and 1 note.)