In the present paper a large corpus of semantic word selection errors from a severe Wernicke's aphasic is analyzed in accordance with a taxonomy recently proposed in the literature. In an attempt to demonstrate the systematicity of the aphasic substitutions, the errors are classified along a wide range of conceptual spheres. In addition, recent theories of the Russian neuropsychologist A. R. Luria are considered in providing an explanation for lexical selection deficits. Finally, comparisons are made between aphasic errors and similar errors seen in "slips-of-the-tongue" in non-brain damaged normal subjects.