Categorization is a basic principle of knowledge organization in the brain. The goal of the current fMRI study was to compare the neural correlates of thematic (e.g., car - garage) and taxonomic (e.g., couch - bed) categories under automatic processing conditions using auditory-to-visual semantic priming. Behavioral data revealed a priming effect for thematically but not for taxonomically related word pairs. On a neural level, thematically related words led to a left-lateralized temporal activation (superior temporal sulcus), whereas taxonomically related word pairs evoked a right-lateralized frontal activation and within the hippocampus. A direct comparison between both categories revealed enhanced activation for thematically related and response suppression for taxonomically related trials in the left superior temporal sulcus. These results suggest that processing of thematic and taxonomic categories leads to activation of distinct brain areas. The mainly right-lateralized fronto-temporal activation for taxonomic relations suggests increased attention and effort for processing this category. The interaction within the left superior temporal sulcus reflects the processing and retrieval of semantic relations whereby specific memory contents seem to influence the direction of activation.