This study investigated relationships among four variables: two teacher variables, critical thinking ability and frequency of oral expressions of logic; and two student variables, critical thinking ability and geometry achievement. Data were secured from geometry classes using audiotape recordings to tabulate teachers' frequency of oral expression of logic, and from standardized tests as measures of critical thinking ability and geometry achievement. Conclusions included a positive relationship between teachers' critical thinking ability and teachers' frequency of oral expressions of logic, and between students' critical thinking ability and students' geometry achievement. Teachers' critical thinking ability and teachers' frequency of oral expressions of logic were not found to be related to students' or class critical thinking ability, or to students' or class geometry achievement.