This paper presents evidence of an inverse frequency effect in jargon aphasia. The subject (Ms. J. P.) showed a predisposition for low-frequency word production on a range of tasks, including picture naming, sentence completion, and naming in categories. Her real word errors were also striking, in that these tended to be lower in frequency than the target. Reading data suggested that the inverse frequency effect was present only when production was semantically mediated, and it was therefore hypothesized that the effect was at least partly due to the semantic characteristics of low-frequency items. Support for this was obtained from J. P's performance on a comprehension task, which showed that her understanding of low-frequency terms—which she often produced as errors—was superior to her understanding of high-frequency terms. The paper considers possible explanations for these findings.