This paper investigates the production of phonological errors in aphasic naming, examining the relationship between these errors and deficits in comprehension. The predictions of Dell and O'Seaghda's (1991) computational model of speech production were tested by lesioning. The set of lesioned models demonstrated a strong correlation between proportion of phonologically related errors in naming and comprehension accuracy. In contrast, no correlation between proportion of phonological errors in naming and comprehension accuracy was found for a group of fifteen aphasics. This paper also examines monitoring behaviours, such as the presence of self-corrections and interrupted responses, again finding no relationship with auditory comprehension. There was also no evidence for a lexical bias in the phonological errors for these aphasic subjects. Phonologically related errors were argued to be words only by chance; this was supported by the fact that they occurred more frequently on short words than long words, and that the proportion of real word errors was not significantly different to that observed in a pseudocorpus of errors. We conclude that a production-based monitor would be compatible with these results but that any comprehension-based monitor is not reliably employed by some or all of the aphasic subjects examined here. Additionally, these data are incompatible with models of language processing where speech input and output share the same processing components.