The purpose of this study was to compare the motivational determinants of low and high readers within the framework of planned behavior theory in order to explain academic achievement in language and mathematics. Teachers' evaluations in language and mathematics at the end of an academic year comprised the dependent variable. Motivational determinants were examined using the constructs that comprised the theory of planned behavior and goal importance. Participants were 258 elementary school students—46 low and 212 high readers—drawn from 30 elementary schools in northern Greece. Results indicated that the two groups differed significantly, with the low students exhibiting significantly lower perceptions of goal importance, intention to achieve, belief strength, outcome evaluation, and normative beliefs, and eventually performing lower in language and mathematics at the end of an academic year than the high readers. A finer analysis of students' motivational determinants was conducted using Covariance Structural Modeling and EQS 5.7b. The planned behavior model enriched with the construct of goal importance produced an almost perfect fit in both language and mathematics for the low readers. The respective fit for the high readers was also acceptable in both language and mathematics. Although model fit was adequate across samples, different weights and signs linking motivational determinants to achievement partially explained the observed differences in language and mathematics manifested in the two groups.