Blatticola chydaeus n. sp. (Nematoda, Thelastomatidae), parasitizing Anurogryllus muticus (De Geer, 1773) (Orthoptera, Gryllidae) in Argentina is morphometric and cytognetic described. It characterized by the female with cuticle annulated up to the middle of the body, the mouth surrounded by eight cephalic papillae, the stoma short with two plates and one tooth, the amphid pore shaped, the oesophagus divided into three parts, anterior cylindrical corpus, isthmus distinct, and basal bulb valvated, the excretory pore located posterior to basal bulb, the nerve ring situated at the beginning of the esophagic isthmus, the intestine broadest anteriorly, the egg oval, smooth shell, in apical view show a triangular section. The tail appendage is short and conic, and presents anal alae. The vulva is protruding, the vagina is long and posterior. The male has one preanal papilla, one simple papilla...