This study was designed to evaluate the effect of the use of mathematical logic in high school geometry on (1) achievement of students in geometry, (2) achievement of students in reasoning in geometry, (3) critical thinking of students, and (4) attitude of students toward logic deduction, and proof in mathematics. The experimental group of six classes participated in a four-week unit in mathematical logic and applied the logic in the learning of deductive methods in geometry. The control group of six classes was taught the same course in geometry without the study of mathematical logic. The findings of this study appear to support the following conclusions: (1) Mathematical logic is an appropriate area of study for both high and average achievers in high school geometry; (2) there is no loss of achievement in geometry caused by devoting time from the traditional course in geometry to the teaching of mathematical logic; (3) including instruction in mathematical logic in high school geometry does not result in a course which is significantly superior to the traditional course in its over-all effect upon student achievement in reasoning in geometry, critical thinking ability or attitude of students toward logic, deductive thinking, and proofs in mathematics; and (4) including instruction in mathematical logic appears to produce a more effective treatment of high school geometry with high achieving students in its effect upon student achievement in reasoning in geometry. (RP)