This study was designed to: (1) investigate the relationships between internal-external (I-E) locus of control and academic achievement for boys and girls over a nine-month period and (2) to determine whether the moderate ability of I-E scores to predict academic achievement is due to the moderate reliabilities of the I-E measures. A total of 279 third-grade children were tested on I-E control and academic achievement in September and again in May. The I-E control measure used was the Intellectual Achievement Responsibility (IAR) questionnaire which yields one subscore for belief in internal responsibility for success (I+) and one subscore for internal responsibility for failure (I-). The achievement measures used in this study consisted of the reading and arithmetic batteries of the California Tests of Basic Skills. The data were analyzed using two sets of canonical correlations: the first set testing the ability of the observed I-E scores to predict later achievement, and the second set testing the ability of these I-E scores statistically corrected for measurement error to predict the same achievement scores. The comparison of these two analyses revealed that improvements in measures of the I-E trait could result in an important predictor of academic performance for boys. (JMB)