Nematodes of the genus Phasmarhabditis Andrássy, 1976 (Secernentea: Rhabditida: Rhabditidae) are terrestrial gastropod parasites and mainly target land slugs. In this study, land slugs were surveyed at 14 locations in seven prefectures of Honshu Island, Japan, to determine Phasmarhabditis spp. infection. Juvenile nematodes of unknown species were found in Meghimatium bilineatum Benson, 1842, at five of the 14 locations. The prevalence and mean intensities ranged from 4.5% to 93.3% and from 4.7 to 22.5 nematodes per host, respectively. A total of 881 juveniles were incubated with slug tissues for 2–10 days, and subsequently developed into adult stage showing the diagnostic characteristics of Phasmarhabditis spp. No nematodes were found from slugs of the genus Lehmannia sampled where M. bilineatum were infected, indicating a difference among host species in their sensitivity to the nematodes. Twenty M. bilineatum from Meguro in Tokyo, Japan, where the prevalence in the host population was >90%, were maintained under laboratory conditions. After 23 days, 11 of the 20 slugs died and the cadavers were infected with numerous nematodes. Since three species of Phasmarhabditis nematodes are known to be lethal to terrestrial gastropods, the nematodes we sampled possibly have a lethal effect on the host slugs.