This study examines whether and how group work affects students` knowledge gathering in mathematics. The focus of the study is based on unprepared group work and how pupils collaborate in work groups. Two tests of students in grade five in Swedish public schools and supplementary observations of their work in groups were carried out. The results of the study show that with respect of the test result, there is no general correlation between those working in groups and those who worked individually. Observations show how students organize themselves in group work without guidance from teachers and how this gave expression in their knowledge development. The study was conducted in four classes during the autumn semester of 2017.