This study was designed to examine the effects of participation in regular outdoor schoolyard environmental education activities on environmental knowledge, attitudes, behaviors, and comfort levels of fourth- and fifth-grade students. Results showed that the fifth-grade treatment group showed statistically significant differences in the measures of environmental knowledge and comfort levels when compared to the control group. There were no significant differences between the fourth-grade treatment and control groups. In addition, significant gender differences were found in the fifth-grade treatment and control groups. The females showed significantly higher scores in the measures of environmental attitudes and behaviors when compared with the males. The significant differences in the fifth-grade treatment group with regard to environmental knowledge and comfort levels in the outdoors, as well as the significant gender differences in the fifth-grade treatment group with environmental attitudes and behaviors, indicate the potential for the effective use of the schoolyard for environmental education activities. (Contains 2 figures and 1 table.)