The purpose of this study was the determination of the effects of fluid and crystallized intelligence and related individualized treatments on achievement of mathematical concepts. One hundred eighty sixth-grade students from three schools participated in the study. All students were given a measure of crystallized intelligence (the S.R.A. Primary Mental Abilities, Revised Test), a measure of fluid intelligence (I.P.A.T. Culture Fair Intelligence Test) and pre- and posttests on mathematical concepts. Subjects were randomly assigned to groups: group 1 received treatment capitalizing on crystallized intelligence; group 2 received treatment capitalizing on fluid intelligence; group 3 received neither treatment. Data analysis was performed on three independent variables (crystallized intelligence, fluid intelligence, sex), one dependent variable (post-test score), and one control variable (pre-test score). Procedures applied were analysis of covariance, analysis of variance, Tukey's post hoc procedures, and t-tests. These analyses revealed significant achievement differences and interaction between treatments and fluid intelligence. Students scoring in the upper one-third on either intelligence measure achieved well regardless of treatment. Students scoring in the lower two-thirds on fluid intelligence achieved more when treatment procedures coincided with their abilities. No differences were found using sex as the independent variable. (Author/SD)