A nematode collected from Diaprepes abbreviatus was identified and described as a new species, Steinernema diaprepesi n. sp. The new species can be distinguished from other species of the genus by the following characteristics: Males: Spicule averaging 79 (71-90) Fm; the ratio D % (distance from the anterior end/esophagus length100) about 80; the ratio SW (spicule length/anal body width) about 1.8. Females: Vulva with short, double flapped epiptygma; tail terminus usually with 5 mucrons. Infective juveniles: Body averaging 1002 (880-1133) Fm, EP (distance from anterior end to excretory pore)=74 (66-83) Fm; tail length=83 (65-91) Fm, and E % (EP/tail length)=89.6 78-114). Lateral field pattern variable, the formula for the arrangement of ridges from head to tail is: 2, 6, 7, 8, 4, 2. The portion with 8 ridges is the longest. This new species can be differentiated from the 3 closest species, S. feltiae, S. glaseri, and S. oregonense by characteristics of their sequences.