This is a report of an experiment performed to evaluate the effects of the teaching of non-decimal numeration systems in grades four and six. Eighteen classes consisting of 430 students comprised the sample. The students were divided into four randomly assigned treatment groups for each level. Tests were given before a teaching session of five to six weeks, again directly following the teaching period, and once more as retention tests seven weeks after the conclusion of the teaching period. Equal arithmetic reasoning group mean scores were achieved by all groups on the posttest. The non-decimal groups of both grades had significantly higher group means on the retention test. Retention was greater among sixth graders than among fourth graders but this knowledge did not improve students' ability to answer questions on place value. Non-decimal group test scores on the posttest showed significant correlations with arithmetic reasoning and computation test scores. The recommendation is made that this topic should be taught in the upper elementary grades and that place value and the relation to the decimal system should be stressed. (Author/FL)