The psychological reality of an abstract consonant dissimilation constraint is demonstrated in an experiment with native speakers of Jordanian Arabic. In this experiment, novel verbs containing constraint violations and those without violations were presented orthographically for judgments of well-formedness. Native speaker well-formedness judgments reflected knowledge of the phonotactic constraint. Systematic gaps were rated much less wordlike than accidental gaps that were equivalent in their lexical characteristics. Judgments for novel verbs containing constraint violations were also gradiently influenced by consonant pair similarity. The experimental study supports previous dictionary-based phonotactic analyses that propose that the native speaker's knowledge of consonant cooccurrence constraints in Arabic is based on emergent generalizations over the lexical items in an abstract root lexicon.