This study examined the relationship between environment-based education and high school students critical thinking skills and disposition toward critical thinking. Four hundred four 9th and 12th grade students from 11 Florida high schools participated in the study. A Pretest-Posttest Nonequivalent Comparison Group Design (9th grade) and a Posttest Only Nonequivalent Comparison Group Design (12th grade) were used. Interviews of students and teachers were used in the classic sense of triangulation. Data collection took place over the 2001-2002 school year. When controlling for pretest score, grade point average (GPA), gender, and ethnicity, environment-based programs had a positive effect on 9th grade students critical thinking skills (p=0.002). When controlling for GPA, gender, and ethnicity, environment-based programs had a positive effect on 12th grade students critical thinking skills (p less than 0.001). The results of this study support the use of environment-based education for improving critical thinking and can be used to guide future implementation. (Contains 3 tables.) [This article reprinted from "Environmental Education Research" (2004) 10(4), pp. 507-522.)