Functional response and mutual interference of Cotesia vestalis (Haliday) was studied on second-instar diamondback moth, Plutella xylostella (L.), larvae. The parasitoid exhibited Type II functional response to its host with attack rates of 0.045 (Holling disc equation) and 0.122 (Rogers random equation) and a handling time of 0.85 h (Holling disc equation) and 1.18 h (Rogers random equation). Theoretical maximum number of hosts that could be parasitized over a period of 24 h by the parasitoid was estimated to be 28.8 (Holling equation) and 20.3 (Rogers equation). Per capita number of parasitization decreased significantly from 12.4 to 3.8 with the increase in the parasitoid density from 2 to 8, indicating that mutual interference occurs when two or more numbers of parasitoid females forage in the same arena. The relationship between searching efficiency (E) and density (P) of the parasitoid was log E = –0.205 0.769 log P, with a mutual interference coefficient of 0.77.