Pathogenicity of Steinernema carpocapsae GSN1 strain was evaluated against Morophagoides moriutii larvae. In Petridishtests, insect mortality by the nematode was dose dependent, which increased with dose from 5 to 160 infectivejuveniles(IJs)/larva. Pathogenicity against fourth-instar larvae was higher than the rate of corresponding second- and third-instarlarvae, showing 100% insect mortality with the dose of 40 IJs/4th instar larva and 80 IJs/2nd or 3rd instar larvae. Lethal concentrationvalues at 50% (LC50) of S. carpocapsae GSN1 strain were 4.2 IJs/2nd instar larva; 8.5 IJs/3rd instar larva; and 2.3 IJs/4th instar larva,respectively. The number of nematodes established in M. moriutii larvae after infection increased in the increment of dose andinsect developmental stage. The highest number of nematodes was harvested from fourth instar larvae of M. moriutii at a dose of160 IJs per larva, showing 22.5 nematodes per insect larva. Nematode reproductive capacity was related to insect developmentalstage, showing 6,335 IJs/2nd instar larva, 21,660 IJs/3rd instar larvae, and 88,700 IJs/4th instar larvae.