Are the phonological representations of printed and spoken words isomorphic? This question is addressed by investigating the restrictions on onsets. Cross-linguistic research suggests that onsets of rising sonority are preferred to sonority plateaus, which, in turn, are preferred to sonority falls (e.g., bnif, bdif, lbif). Of interest is whether these grammatical preferences constrain the recognition of auditory and printed words by speakers of English--a language in which such onsets are unattested. Five experiments compare phonological lexical decision responses to nonwords, including unattested onsets, through either aural or visual presentation. Results suggest that both hearers and readers are sensitive to the phonotactics of unattested onsets. However, the phonotactic generalizations of hearers and readers differ on their scope and source. Hearers differentiated all three types of onsets (e.g., bnif, bdif, and lbif), and their behavior implicated both grammatical and statistical constraints. In contrast, readers were able to differentiate only those structures similar to attested English onsets from dissimilar structures (i.e., bnif vs. bdif or lbif), and their preferences reflected statistical knowledge alone. These findings suggest that the phonological representations informing lexical decision to spoken and printed words are not isomorphic.