Despite its importance as a central goal in environmental education, there appears to be little consensus about how best to document, assess and evaluate understanding of environmental concepts. This illustrative case study describes and demonstrates the use of the concept mapping strategy as an effective tool for assessing environmental understanding. Data were collected from 325 middle school-aged (11-14 years) individuals and comparisons were made among those enrolled in a model, university-based, informal, residential marine education program (viz. "MarineQuest") and a matched group of non-participating students. Using a non-randomized Solomon Four Group experimental design, differences in the structural complexity and content validity of knowledge about marine animal life were explored. Results reveal significant differences in the frequencies of concepts, relationships, levels of hierarchy, branching and cross-linking and in the frequencies and types of critical concepts depicted. In all cases, where differences were found, they favored participants who enrolled in the environmental education intervention. The findings suggest that concept maps offer a valuable alternative or adjunct to traditional pencil and paper tests and provide both a qualitative and a quantitative measure of conceptual understanding. (Contains 12 figures.)