Reported is an investigation of: (1) the effect of teacher wait-time on science achievement; and (2) the difference in the achievement of children at the formal stage and those at the concrete stage of cognitive development. Teacher wait-time was defined as the length of the pause preceding any teacher utterance. The sample consisted of 23 intact classes of children from grades 5, 6 and 7 in Australian schools. Three treatment groups were used: (1) wait-time greater than 2-7 seconds; (2) wait-time greater than one second but less than 2-7 seconds; and (3) wait-time less than or equal to one second. Science achievement was measured with paper and pencil tests based on physical science concepts taught in the study. Results showed that the extension of teacher wait-time produced desirable changes in science achievement for fifth, sixth, and seventh graders. (HM)