Allocreadium neotenicum sp. nov. is described from dytiscid water beetles, where specimens occur unencysted in the haemocoel and develop precociously to fully mature adults with large number of eggs. The adults have a well developed stylet and intact eyespots, neotenic structures which demonstrate that the larva is an ophthalmoxiphidiocercaria. The genus Allocreadium is analyzed and an emended diagnosis is proposed. The systematic position of all species currently included in the genus is discussed. A. wallini and A. chuscoi are transferred to Crassicutis, A. oncorhynchi and A. boleosomi to Plagioporus; P. lepomis is reduced to synonymy with P. boleosomi. Other species are removed from Allocreadium but are not assigned specifically to other genera.