Prothallonema tomici n. sp. (Tylenchida: Sphaerulariidae) is described as a parasite of the bark beetle Tomicus piniperda (Linnaeus) (Coleoptera: Curculionidae: Scolytidae). The new species is characterised by the relatively short body (0.3-0.5 mm) and short prolapsed uterus ((0.16-0.44) × (0.065-0.15) mm), relatively anterior position of the vulva (83.8-85.5%), stylet with fine asymmetrical basal knobs, hemizonid and hemizonion, four incisures, subterminal phasmids and tail terminus always curved dorsad. The entomoparasitic and mycetophagous generation were observed in the body cavity of T. piniperda. Comparing the two generations of J4 showed that the mycetophagous generation had a longer and more robust body, shorter pharynx and tail, longer reproductive system and vulva located further posteriorly. It can be concluded that, in the haemocoel, the entomoparasitic generation can develop through all stages whilst the mycetophagous generation develops to the J4 and then dies.