One female of Culex fatigans Wied., 1828, was collected in a light trap at CENA-ESA«LQ»-USP, Piracicaba, at about 500 meters off the Piracicaba River banks. This mosquito exhibited 15 partially engorged pionid larvae of water mites (Prostigmata, Hydrachnellae) of the genus Arrenurus, attached to the articular membranes of the abdominal segments; no mites were found on the thorax or coxae. The larva is drawn, dorsad and ventrad. Since species cannot be identified from larvae, no specific name is given.