It is a well known fact that co-articulation is one of the many sources of variability that affects phonetic category structure and ultimately the perception of the phonetic categories of speech. What is less clear is what influence, if any, phonetic category structure has on higher levels of language processing and, in particular, on accessing the mental lexicon. A series of experiments are reviewed exploring the effects of acoustic-phonetic variants of a phonetic category on lexical processing. Specifically, these studies investigated the extent to which modifications in the voice-onset time of initial voiceless stop consonants of prime stimuli affected the magnitude of semantic priming in a lexical decision task. The results of these studies allow for the following conclusions: acoustic-phonetic structure influences lexical activation; the prototypicality of an exemplar member of a phonetic category influences the degree of lexical activation; and acoustic-phonetic structure not only affects the activation of its associated lexical representation and lexical network, but also the lexical representation and lexical-semantic network of its competitors.