A new endoparasitic mite, Novophytoptus longissimus Chetverikov et Petanović n. sp., living in parenchymatous stem tissue of the rush Juncus conglomeratus L. (Juncaceae) is described with the aid of conventional light microscopy, confocal light microscopy and scanning electron microscopy. Immatures of the new species lack eye-like structures (sensu Smith 1977), which are well-developed in adults. Adults possess unique globular protuberances on the dorsal surface of coxae I, uncommon localization of gnathosomal seta ep (situated in a pocket-like space formed by an antero-dorsal fold of the palpcoxa) and some uncommon attributes in the male and female genital areas. In one male the two putative podocephalic canals (sensu Nuzzaci & Alberti 1996) and the disjointed digitus mobilis and digitus fixus of the chelicerae were clearly seen under DIC LM. Senescent females of N. longissimus n. sp. exceed 510 micrometers, making this species the longest among contemporary eriophyoids. Remarks address the size limits of eriophyoids and the possible ways that novophytoptines penetrate the epidermis. Microphotographs of N. longissimus n. sp. moving within the aerenchyma of the rush stem are presented for the first time. The term “vermiformity”, referring to the elongation of the mite body, is discussed and width/length ratio is proposed as a precise estimate of “vermiformity”.