This study investigated whether abstract phonotactic constraints play a role in speech processing. Dutch listeners performed an auditory lexical decision task, in which the nonword stimuli either did or did not violate a phonotactic constraint. Listeners were faster to reject nonwords that violated a phonotactic constraint. This effect remained significant even after partialling out the effects of lexical factors, such as the similarity of the nonwords to existing words in the lexicon. This finding constitutes, to our knowledge, the first demonstration of the involvement of pure abstract phonotactic constraints in on-line speech perception.