Most current models of knowledge organization in the brain are based on hierarchical or taxonomic categories (animals, tools). Another important organizational pattern is thematic categorization, i.e. categories held together by external relations, a unifying scene or event (car and garage). We used fMRI to examine neural activation patterns as subjects performed a category construction task where these two category types were contrasted. Subjects were visually presented with a target word followed by the presentation of two match words and had to choose by button press one match that goes best with the target word. In the balanced or cross-categorization condition (Car/Garage Bus) both match words fit the target; in the biased conditions only one match word fit the target either thematically (Car/Garage Brush) or taxonomically (Car/Bus Eraser). We found that in the biased conditions, thematic and taxonomic categories recruited very similar cortical regions: left inferior frontal, middle temporal and occipital regions. In the balanced condition subjects showed no behavioral preference for either thematic or taxonomic matches. However, contrasting signal changes during a subjective taxonomic choice in the presence of a thematic alternative vs. a subjective thematic choice in the presence of a taxonomic alternative required the additional recruitment of right middle frontal gyrus, left precuneus and left thalamus. Our results suggest that thematic relations between objects are processed similarly to taxonomic relations, but require less cerebral processing demand, providing validation for thematic categories as an alternative principle of conceptual organization.