The purpose of this study was to determine what representations of content existed in a secondary environmental science class and what happended to those representations during curriculum occasions. Initial data construction involved attention to what was actually going on during class sessions. Following this, a reanalysis of the data corpus with specific focus questions allowed the writing of descriptions of each occasion for content, and these descriptions became the units of analysis in the study. Some class sessions involved occasions for two or three pieces of content being tracked; others involved just one. These occasions were analyzed separately. Also, an analysis was conducted across occasions in order to track what happened to pieces of content across time and sessions. The findings that emerged from the analysis across occasions was especially informative and useful. A major piece of content tracked was "niche," a fundamental construct in environmental science. The development of niche, which spanned 16 class sessions, began on the first day, moving from an exploration of roles, especially the powerful roles humans have in the ecosystem, and going on through the various representations of content of niche, population change, and entropy as each evolved over the course of instruction. The findings indicate that instruction when studied in the contexts in which students experience it, is not an inert instructional variable, but rather that the dynamic nature of content can exist in an experienced curriculum. Therefore, asssumptions about what the content is in a classroom may be vulnerable to much variation. (Contains 20 references). (AA)