An instrument was developed to assess the logical reasoning capacity of adolescents in the content area of environmental science. Characterized as a group test, the instrument consisted of five concrete experiences related to environmental concepts, to which pencil-and-paper responses were made. The instrument was validated through (1) logical analysis of items from the standpoint of Piagetian theory, (2) correlation with clinical interviews and group test results in physical science, and (3) factor analysis. The results indicated an acceptable level of validity for the instrument in each category. Concurrently, the results of the study indicated that the reasoning level of the subjects was well below the theoretical levels predicted for them by theory. The results also suggested that reasoning ability in environmental science was lower than in areas associated with the physical concepts usually tested. The data suggested the presence of a horizontal decalage, or time lag, between the development of logical reasoning skills in areas of familiarity, such as physical science, and areas of content less familiar to the subject or those in which logical structures may be less frequently used in the development of evaluation instruments.