The genus Teratocephalus contains a number of species of which three species occur in the Netherlands. It are small bacterial-feeding nematodes with a strongly annulated cuticle and therefore, as genus, easy to recognize. Traditionally nematodes were grouped in Secernentea and Adenophorea. The first group contains the majority of the plant parasites the famous bacterial feeding Caenorhabditis and many other bacterial feeding nematodes. The family Teratocephalidae, with the genus Teratocephalus has long been considered to belong to the Secernentea but is also shows morphological details which suggest a placement in the Adenophorea. With modern molecular techniques is has been shown that Teratocephalus shows more relationships with the Adenophorea and that the whole classification of Nematoda has to be reconsidered.