Laboratory inoculation experiments were conducted to study the infectivity of Steinernema feltiae (DD-136) on fed and unfed larvae of common cutworm, Spodoptera litura, of different body sizes. All larvae used were killed within 48 hr after inoculation with ca. 1, 000 infective juveniles. With increasing body weight, longer time was required to kill the larvae : the average body weights of sluggish, moribund, and dead larvae 24 hr after inoculation were about 390, 280, and 200 mg, respectively. Nematode infectivity on fed and unfed larvae became considerably different with decreasing inoculum size of nematodes from 500 to 5 per insect. Number of invading nematodes in dead larvae ranged from 1 to 6 per mg body weight of insect, while that in living ones was less than 1. Similarly, larvae which fed other larvae under crowded conditions were infected with fewer nematodes. Generally, nematode infection through the anus was less in the fed larvae than in the unfed. Average number of symbiotic bacteria, Xenorhabdus nematophiluss, isolated from 10 μl hemolymph 24 hr after inoculation was 3×10⁴ for dead larvae and only about 5 for those still living, including the sluggish. The susceptibility of S. litura larvae of different sizes was discussed from the standpoint of the nematode infection process.