This study was concerned with identification of elementary school grade levels appropriate for teaching various phases of the metric system. The same unit was taught to 201 elementary school children in grades three through six. Pretests and posttests were used to assess progress made by students in developing skills and understandings related to the metric system. Comparison of grade levels was accomplished by analysis of covariance with 0.01 chosen as the significance level. Differences in grade level had no significant effect in achievement for children's learning the metric system concerning understanding the meaning and approximate sizes of metric units, understanding use of measurement equipment, understanding the organization of the metric system, and the ability to measure length in the metric system using whole numbers. Achievement of grade three was significantly lower than other grades on the ability to measure liquid volume and weight using whole numbers. Grades five and six achieved significantly higher on conversion between metric units, on determination of area and cubic volume, on measurement of length, liquid volume and weight using decimals and on conversion between metric units using decimals. Highest overall achievement in learning the metric system was attained by pupils in grade five. (Author/JP)