Abstract Presence of the genus Neoparasitylenchus Nickle, 1967 (Tylenchida, Allantonematidae), parasitizing Harmonia axyridis (Pallas, 1773) (Coleoptera, Coccinellidae) in Argentina. We describe a new species Neoparasitylenchus platense sp. n., which is characterized by having the free living stages lip region flatly rounded, spear slender, faintly knobbed, tail rounded, females with light brown body, stylet not sunken into body, and not prominent, subterminal vulva and anus, obtusely rounded tail end, with a peg-like projection, oviparous, and the males with a little stylet and partially degenerate oesophagus, spicules filiform with widened base, short gubernaculum, leptodera bursa.

Resumen Se cita por primera vez el género Neoparasitylenchus Nickle, 1967 (Tylenchida, Allantonematidae), parásito de Harmonia axyridis (Pallas, 1773) (Coleoptera, Coccinellidae) para Argentina. Se describe la especie nueva Neoparasitylenchus platense sp. n. que se caracteriza por tener en las formas de vida libre la región labial rodeada de una estructura plana, estilete delgado, levemente curvado, apéndice caudal redondeado, hembras con cuerpo marrón claro, estilete no hundido en el cuerpo y no prominente, vulva y ano subterminal, apéndice caudal redondeado, con una proyección en forma de clavija, ovíparos. Los machos con un estilete y un esófago parcialmente degenerados, espículas filiformes con base ensanchada, gubernaculum corto, bursa leptodera.

Three new species of Cephalobium Cobb, 1920, C. laplata from City Bell, C. polidentatum from Lincoln and C. dispar from Gorina, parasites of Gryllodes laplatae Saussure, 1877 from Buenos Aires, Argentina, are described and illustrated. Cephalobium laplata can be differentiated by having the cheilostom with a dorsal unmovable tooth, telostom with three ventral little teeth and two ventral movable claw teeth and gubernaculum triangular and five pairs of genital papillae. Cephalobium polidentatum has cheilostom with a movable ventral tooth, prostom with four dorsal movable teeth and telostom with three teeth; gubernaculum triangular with projections and with one pair of preanal and six pairs of postanal papillae. Cephalobium dispar is characterized by having telostom with two wings around the spicules and one pair of preanal and six pairs postanal papillae.

Cameronia laplatae n. sp. found in City Bell, Argentina, parasitizing crickets is described and illustrated. The new species is distinguished from other members of the genus Cameronia by the distribution pattern of the genital papillae in the male. It differs from all congeners mainly in having two pairs preanal y two pairs postanal papillae

Cephalobium bidentatum sp. nov., parasitizing nymphs of Gryllodes laplatae Sauss (Orthoptera, Gryllidae) from Argentina, is described and illustrated. It is distinguished from other members of the genus Cephalobium COBB, 1920, by having the buccal cavity very sclerotized with two hook-shaped teeth, vagina short and muscular, male has two spicules with hook-shaped tips, and by the distribution pattern of the postanal papillae: one pair under the anus, three pairs between the anus and the tail, and two pairs at the base of the tail appendage.