Knowledge about word and object meanings can be organized taxonomically (fruits, mammals, etc.) on the basis of shared features or thematically (eating breakfast, taking a dog for a walk, etc.) on the basis of participation in events or scenarios. An eye-tracking study showed that both kinds of knowledge are activated during comprehension of a single spoken word, even when the listener is not required to perform any active task. The results further revealed that an individual's relative activation of taxonomic relations compared to thematic relations predicts that individual's tendency to favor taxonomic over thematic relations when asked to choose between them in a similarity judgment task. These results indicate that individuals differ in the relative strengths of their taxonomic and thematic semantic knowledge and suggest that meaning information is organized in 2 parallel, complementary semantic systems. (Contains 2 figures, 3 tables and 2 footnotes.)