This study investigated the effects on meaningful learning achievement of concept-related instructional organization and locus of control orientation. Two five-week instructional treatments were developed, one which explicitly stressed concept relationships and one which did not. Five hundred and forty-one subjects at six Indiana high schools were involved in the study. Analyses of covariance of posttest and six-week retention test scores with treatment as the independent variable showed no significant treatment effect. Analyses of variance of test scores with locus of control orientation as the independent variable resulted in highly statistically significant differences. Subjects with an internal locus of control orientation achieved more than externally oriented subjects. A two-way analysis of covariance by treatment and locus of control orientation resulted in a statistically significant treatment/locus of control interaction effect for retention test scores. Externally oriented subjects who were exposed to the concept-related treatment generally retained more than those in the comparison group. Internally oriented subjects retained about the same amount of information regardless of treatment group. When the male and female subjects were analyzed separately, the interaction effect appeared greater for females than for males.