There is growing agreement that people with dyslexia have difficulty processing speech sounds. Proponents of the phonological deficit hypothesis argue that the core difficulty in dyslexia lies with phonological processing itself, either in encoding or access to representations, or in underspecified or ‘fuzzy’ phonological representations. Here, we focussed on the speech of dyslexic people to investigate whether the phonological deficit is evident in their inner and overt speech errors. We considered their tendency to make errors with produce real words (lexical bias) and which involve substitution of similar phonemes (phonemic similarity effect), hypothesising that dyslexics would not show a phonemic similarity effect but may show a lexical bias. We used a tonguetwister paradigm to elicit substitution errors in inner and overt speech. 16 dyslexics recited 4-word tonguetwisters, aloud or silently, and reported any errors that they perceived themselves to make. We manipulated the onset similarity of the tonguetwisters and whether a substitution would result in either a word or a nonword. We found evidence for both a similarity effect and a lexical bias, and our results suggested that dyslexics did not have a severe difficulty monitoring their own speech. Implications for theories on the causes of dyslexia are discussed.