Conducting and interpreting an interview is more problematic when informants use a word that has multiple meanings and interpretations. In this case, the problematic word, “environmentalist,” labeled several socially-defined identities that were central to the study. The analysis is based on interviews with 156 members of 20 diverse environmental groups (and two comparison groups) in the Eastern United States, including their views on environmentalists, their history with the movement, their self-identification as an environmentalist, and their environmental actions. From these data, principles of classification and naming are used to distinguish the multiple meanings of the identity “environmentalist.” We found that informants use the term to describe four distinct types of people: 1) those who say they care about the environment but take no public actions; 2) those who act to preserve local habitat often through private actions (also called “conservationists”); 3) those who act in the civic or political realm, by writing to representatives or attending hearings (also called “activists”); and 4) those who act via demonstrations, civil disobedience, or “direct action” such as blocking logging operations (also called “radicals”). These differing meanings are sometimes used strategically by participants to position themselves, or opponents, within the environmental movement. The polysemy of the word environmentalist renders it a poor choice for questions in surveys and interviews unless disambiguating paraphrases are added. Additionally, cross-tabulation shows that named environmental identities are indicators of behavior—self-defined environmentalists also reported significantly more environmental actions. Words or paraphrases that distinguish among the multiple meanings of “environmentalist” further improve these identity terms as predictors of behavior.