The effects of auditory selective attention on event related potentials (ERPs) to speech sounds were examined in subjects attending to vowel-consonant-vowels (VCVs) in one ear while ignoring VCVs in the opposite ear. In one condition, subjects discriminated phonetic changes in the VC, CV, or both formant-transition regions. In another condition, they discriminated equally difficult intensity changes in the same VCV regions. Attention-related negative difference waves showed enhanced early and late components (Nde and Ndl) during phoneme-discrimination conditions. Hemispheric asymmetries developed only during the Ndl and were more pronounced during phoneme discrimination. The results suggest that auditory areas of both hemispheres are specialized for phonetic analysis, with hemispherically specialized mechanisms engaged primarily during the final stages of phoneme processing.