Examination of the ultrastructure of spermiogenesis and mature sperm in six species of Temnocephalidae revealed the origin of the spiral region of microtubules in the sperm shaft. The region is characteristic of many Temnocephalida but absent from other platyhelminth taxa. During spermiogenesis, an electron-dense heel region develops from, or adjacent to, the outer plates of the intercentriolar body in the zone of differentiation. The intercentriolar body splits into two halves, each of which remains attached to one basal body via the dense heel. The entire anchoring apparatus of each flagellum, consisting of the basal body, striated rootlet and the intercentriolar body and dense heel, rotates around the spermatid shaft until the two basal bodies lie parallel to each other. This rotation causes the compression of one of the two semi-circular rows of microtubules which originate on opposite sides of the spermatid shaft. The row is compressed to a tight horseshoe shape with electron-dense material between the folded halves. Distal to the flagellar insertion region, the shaft tapers and the number of microtubules decreases. Proximally the inner folded row gradually opens to join the remaining semicircle of microtubules by one end joining the outer array first. The other end remains inside the peripheral row for some distance, resulting in a spiral arrangement along part of the shaft. In the six species examined, the proximal end of the sperm terminated either in an extensive flange that contained only microtubules or by the shaft splitting into a number of fine processes, each containing only one or a few microtubules.