Gyrodactylus rysavyi, a monogenean parasite of the skin, fins and gills of the Nile catfish, Clarias gariepinus, is capable of directional swimming when detached from the host and released in open water. The parasite propels itself by vigorously bending the body into a loop and then unbending the body with equal vigour. A typical swimming phase lasts for 2-6 sec, involves between 4 and 8 looping/unlooping actions per sec and propels the parasite in any direction, including vertically upwards or downwards, at speeds of 1.7-5 mm/sec. The parasite is capable of swimming upwards for a distance of at least 15 cm. At the end of each swimming phase, the parasite sinks slowly, performing while it does so twisting and turning movements. The duration of this resting phase is similar to that of the swimming phase. A unique feature of the haptor of G. rysavyi is a posterior shift in the position of the 16 hooklets relative to the two large hamuli. The long handles of the hooklets radiate outwards, and like ribs support the fan-shaped posterior region of the haptor. This arrangement may be a specialization related to the adoption of looping and unlooping swimming movements, since the hooklet-supported fan is likely to provide most of the propulsion during swimming. Consideration is given to the possible role of swimming and the twisting and turning behaviour of the passively sinking parasite in the dispersal and transmission of G. rysavyi. The gyrodactylids Macrogyrodactylus congolensis and M. clarii, which also parasitize C. gariepinus, do not swim when detached from the substrate.