Abstract: Interspecific competition between two major parasitoids, Diadromus collaris and Oomyzus sokolowski, of the diamondback moth, Plutella xylostella, was studied in the laboratory at 26℃. D.collaris females attacked and inserted their ovipositor into diamondback moth pupae that had been parasitized by O.sokolowskii since early 2nd and 4th instars, but refrained from oviposition into the hosts. The duration of ovipositor insertion was shorter (3-5s) in previously parasitized pupae than in unparasitized pupae (5-8s). The frequency of D.collaris ovipositor insertions into both parasitized and unparasitized pupae was similar, but it was reduced with the increasing age of O.sokolowskii immatures developing inside the host pupae. Ovipositor insertion by D.collaris did not reduce the survival of O.sokolowskii immatures inside the host pupae. Compared with instances when only unparasitized pupae were provided, the parasitism rate by D.collaris was reduced significantly by the presence of host pupae already parasitized by O.sokolowskii, but the sex ratio of the progeny produced by D.collaris was unaffected, when parasitized and unparasitized pupae were provided concurrently. However, the combined rate of parasitism by the two parasitoids was higher compared to that by one of the parasitoid.