To examine the effects of stimulus structure and variability on perceptual learning, we compared transcription accuracy before and after training with synthetic speech produced by rule. Subjects were trained with either isolated words or fluent sentences of synthetic speech that were either novel stimuli or a fixed list of stimuli that was repeated. Subjects who were trained on the same stimuli every day improved as much as did the subjects who were given novel stimuli. In a second experiment, the size of the repeated stimulus set was reduced. Under these conditions, subjects trained with repeated stimuli did not generalize to novel stimuli as well as did subjects trained with novel stimuli. Our results suggest that perceptual learning depends on the degree to which the training stimuli characterize the underlying structure of the full stimulus set. Furthermore, we found that training with isolated words only increased the intelligibility of isolated words, although training with sentences increased the intelligibility of both isolated words and sentences.