To investigate the efficacy of a training procedure designed to facilitate the attainment of the mathematical concept of number conservation, 43 children from grades 1 and 2 were tested for their ability to conserve. Some of the pupils were found to be nonconservers; some, transitional conservers; and some, conservers. Pupils of the first two groups were divided into a training and control group. The training group received special sessions dealing with number conservation; the control group did not. It was found that although the older children had more initial conservers, both the younger and older children responded similarly to the training program. The training group children all learned how to conserve. The control group remained generally unable to perform conservation tasks. Retesting indicated that the abilities gained from the training sessions were surprisingly permanent. (WD)