Cotesia vestalis Haliday (Hymenoptera: Braconidae) is a solitary koinobiont endoparasitoid that causes high larval mortality of Plutella xylostella L. (Lepidoptera: Plutellidae) on Brassica crops in South Africa. Many farmers use prophylactic and curative microbial insecticides such as Bacillus thuringiensis (Berliner) var. kurstaki (Btk) to minimise damage caused by P. xylostella on their crop. Bacillus thuringiensis formulations are widely adopted based on them not having a direct effect on parasitoids, which then assumes that they complement each other in pest management. We investigated parasitism of P. xylostella by C. vestalis in cabbage fields treated with Btk and assessed the implications on cabbage yield. Populations of P. xylostella and C. vestalis were monitored for 12 weeks in each of the four growing seasons in Btk-treated and control plots at Alice, Eastern Cape, South Africa. Plutella xylostella densities and levels of parasitism were lower in the Btk-treated plots. Consistent with low infestation, damage in Btk-treated cabbage was significantly lower than in the control.