The purposes of this descriptive-predictive study were to investigate the logical and critical thinking abilities of a sample of sixth through twelfth grade students (N=173) and to determine whether logical thinking processes are predictors of critical thinking abilities and academic achievement. The instruments administered in this study were (1) Group Assessment of Logical Thinking (GALT); (2) Watson-Glaser Critical Thinking Appraisal; (3) Ross Test of Higher Cognitive Abilities; (4) SRA; and (5) MAT6. Although significant gender differences were not found for the total scores on the GALT, Ross, and Watson-Glaser, a significant gender difference in favor of the males was found for probabilistic reasoning on the GALT. The five formal reasoning modes on the GALT were predictors of critical thinking as measured by the Ross and the Watson-Glaser. Also, the formal operational modes on the GALT were significant predictors of academic achievement. The results of this study indicated that a significant percentage of students in grades six through twelve were neither logical nor critical thinkers. (Author/TW)