The efficacy of dominant Steinernema siamkayai Stock, Somsook and Reid isolated from Karaikal region was tested against some of the important crop pests such as Spodoptera litura, Plutella xylostella, Leucinodes orbonalis, Earais vitella and Cnaphalocrocis medinalis. The dosage and time mortality relationship of S. siamkayai against the third, fifth larval instars and prepupal stage of S. litura indicated that as the dosage increased the susceptibility also increased. The relative susceptibility against the above mentioned stages of the same pest indicated that the exposure time increased the susceptibility of the insect. The same trend was registered in respect of P. xylostella, L.orbonalis, E. vitella, and C. medinalis both for dosage and time mortality relationship and relative susceptibility for different exposure periods.