Hebrew constrains the occurrence of identical consonants in its roots: Identical consonants are acceptable root finally (e.g., skk), but not root initially (e.g., kks). Speakers' ability to freely generalize this constraint to novel phonemes (Berent, Marcus, Shimron, & Gafos, 2002) suggests that they represent segment identity—a relation among mental variables. An alternative account attributes the restriction on identical phonemes to their feature similarity, captured by either the number of shared features or their statistical frequency. The similarity account predicts that roots with partially similar consonants (e.g., sgk) should be at least as acceptable as roots with fully identical consonants (e.g., skk), and each of these roots should be less acceptable than dissimilar controls (e.g., gdn). Contrary to these predictions, three lexical decision experiments demonstrate that full identity is more acceptable than partial similarity and (in some cases) controls. Speakers' sensitivity to consonant identity suggests that linguistic competence, in general, and phonology, in particular, encompass a computational mechanism that operates over variables. This conclusion is consistent with linguistic accounts that postulate a symbolic grammatical component that is irreducible to the statistical properties of the lexicon.