The objective of this study was to develop an instrument to observe the play behaviour of a whole group of children from four to six years of age in a classroom setting on the basis of video recording. The instrument was developed in collaboration with experienced teachers and experts on play. Categories of play were derived from the literature and daily practice in Dutch classrooms (i.e. sensory, motor, construction, make-believe play and arts-and-games). Analysis of the video with the help of the observation instrument showed that the between-observer reliability was almost perfect. The simple and clearly structured instrument may be used by teachers or in teachers' education. (author abstract)