Abstract A series of laboratory assays and tests was conducted to determine the route(s) of parasitism of Phyllophaga hirticula larvae by the entomogenous nematodes Steinernema carpocapsae and Heterorhabditis heliothidis as well as the mechanism(s) responsible for the relative resistance of these grubs to nematode parasitism. Scanning electron microscopy (SEM) showed that the spiracles were cribiform in structure with a spiracular plate punctured with aeropyles that were the only openings to the trachae. These openings were too small (6 × 3 μm) for entry by nematodes measuring 25 μm in diameter. Penetration of the cuticle was not observed. Parasitism was higher when nematodes entered through the mouth rather than the anus. Nematodes penetrated the wall of midguts dissected from Galleria mellonella larvae faster than midgut walls dissected from P. hirticula larvae. Histological studies showed the peritrophic membrane to be much denser in P. hirticula than in G. mellonella and, thus, probably acted as a barrier to nematode parasitism. Video tapes of nematode behavior within excised midguts of G. mellonella showed that penetration of the midgut wall was due to random movements of the nematodes within the midgut. Once a hole was created by a nematode, others also escaped through the opening. However, food passing through the alimentary tract may remove a large proportion of the nematodes prior to penetration of the wall.