To determine the effects of instruction in general semantics on the critical thinking of secondary students, 104 tenth- and eleventh-grade students in control and experimental groups completed the "Watson-Glaser Critical Thinking Appraisal" test before and after the 12-week teaching period, in which two teachers spent 1 hour per week in the instruction of general semantics and 1 hour per week in the instruction of the basic principles of grammar. Results indicated that (1) the average mean score gains of the experimental classes were slightly higher than those of the control classes; (2) at the .05 level of significance, the two groups showed no significant post-test differences; (3) because the control groups improved as much in critical thinking as the experimental groups, the lessons in general semantics alone were not considered a significant factor in critical thinking improvement; (4) the teacher was considered an important factor in the improvement of critical thinking; (5) the two participating teachers indicated their belief that critical thinking was developed in the experimental group, and that general semantics should therefore be included in the secondary school English curriculum. (Author/LH)