Encapsulation is an effective cellular immunity feature in insects, but the mechanism by which hemocytes recognize and encapsulate invading nematodes remains unclear. In addition, insect susceptibility to nematodes differs, indicating differences in immunity. We compared cellular encapsulation with non-parasitic and parasitic nematodes in two lepidopteran insects in vivo and ex vivo. The percentage of the free-living nematode Caenorhabditis elegans (Maupas) (Rhabditida: Rhabditidae) encapsulated following injection into Galleria mellonella Linnaeus (Lepidoptera: Pyralidae) larvae in vivo was low, whereas most larvae were encapsulated in Mythimna separata walker (Lepidoptera: Noctuidae). Similarly, the entomopathogenic nematode Steinernema carpocapsae (Rhabditida: Steinernematidae) was rarely encapsulated in G. mellonella , but > 50% were encapsulated in M. separata . Adhesion of G. mellonella hemocytes on the surface of live C. elegans was infrequently observed ex vivo, whereas dead nematodes were partially covered with hemocytes. A significantly higher percentage of live nematodes was covered with hemocytes in M. separata in the presence and absence of insect plasma compared with G. mellonella . These results indicate that there is a difference in immunity against nematodes between the two insects and that the difference largely depends on the capabilities of the hemocytes.