Pintomyia fischeri adults collected in different eco-epidemiological studies in the northeastern of Argentina were found parasitized by juvenile nematodes (Tylenchida) isolated from the body cavity. The percentage of infected females and males was 3.8% and 2.9% respectively. Part of the life cycle of sand flies and tylenchid nematodes take place in humid and dark sites, where infection of immature stage of Phlebotominae insects is possible. Biology of this parasite could help to determine the breeding sites of sand flies. This study constituted the first report of tylenchid nematodes infecting sand flies at field conditions in South-America.