The purpose of this study is to examine multiple influences on twelfth-grade environmental knowledge for a national probability sample of high school students. A longitudinal model was developed and tested incorporating background factors (socioeconomic status and gender) and intervening variables (internal locus of control, level of science class, and informal science education). The variables were placed in the model such that the developmental process of environmental knowledge from tenth-grade to twelfth-grade could be examined. Results revealed that environmental knowledge is the complex result of several diverse variables working collectively. All of the variables in the model made significant contributions (in terms of direct and indirect effects) to environmental knowledge. Of particular interest was the influence of internal locus of control on twelfth-grade environmental knowledge. Aside from the baseline measure, tenth-grade knowledge, internal locus of control had the strongest direct influence on twelfth-grade environmental knowledge. This finding suggests that individuals with an internal locus of control seem to be motivated to obtain more environmental knowledge than those students with an external locus of control. (Author/LZ)