The objective of this study was to develop a nonformal environmental education program in a summer camp and to measure its effectiveness increasing environmental knowledge and attitudes of the participants. Seventy six teenagers between 14 and 17 years participated. Activities dealt with both natural and urban environment. Preactivity and postactivity surveys were used for the assessment, using Wilcoxon test for paired samples. The results showed that there were no changes in the knowledge and perception of the environment, but important attitudinal differences occurred. We conclude the target was partially reached.