The physiological and histochemical characteristics of the gracilis muscle were studied in 19 patients undergoing electrically stimulated gracilis neosphincter construction. Indications for surgery were faecal incontinence (n = 11) and reconstruction following sphincter excision or congenital absence (n = 8). Transposition of the gracilis muscle around the anal canal followed by chronic low-frequency electrical stimulation was associated with a shift in the frequency-response curve and a prolongation of the time-course of individual muscle twitches suggestive of transformation to a slow-twitch fatigue-resistant type. Temporary cessation of electrical stimulation resulted in a reversal of the frequency-response changes. Muscle biopsies taken before and a median of 80 (range 49–137) days after transposition and low-frequency electrical stimulation indicated a significant increase in the proportion of type 1 fibres and a significant decrease in their diameter. These results show that the human gracilis muscle is capable of physiological and histochemical adaptation to long-term neosphincter function.