Group work has been promoted in many countries as a key component of elementary science. However, little guidance is given as to how group work should be organized, and because previous research has seldom been conducted in authentic classrooms, its message is merely indicative. A study is reported, which attempts to address these limitations. Twenty-four classes of 10-12-year-old pupils engaged in programs of teaching on evaporation and condensation, and force and motion. Both programs were delivered by classroom teachers, and made extensive use of group work. Pupil understanding progressed from pre-tests prior to the programs to post-tests afterwards, and results suggest that group work played a critical role. Organizational principles are extrapolated from the findings, which could be readily adopted in classrooms.