To test the hypothesis that formal operational reasoning modes are predictors of critical thinking abilities and grades assigned by teachers in science and mathematics, in September 1986 the Group Assessment of Logical Thinking (GALT) and in December 1986 the Watson-Glaser Critical Thinking Appraisal (WGCTA) were administered to 101 rural students in Grades 9 through 12. The grades assigned by teachers were collected in May 1987. Construct and criterion-related validities and internal-consistency reliability using Cronbach's alpha method were established on the GALT. On the WGCTA, content and construct validities and internal consistency reliability using the split-half procedure, coefficient of stability, and coefficient of equivalence were established. The five formal operational reasoning modes in the GALT were found to be significant predictors of critical thinking abilities and grades assigned by teachers in science and mathematics. The variance in the five critical thinking abilities attributable to the five formal operational reasoning modes ranged between 28% and 70%. The five formal operational reasoning modes explained 29% of the variance in mathematics achievement and 62% of the variance in science achievement.