Larvae of the diamondback moth (DBM), Plutella xylostella L. (Lepidoptera: Ypeunomutidae), cause serious losses to cabbage, Brassica oleracea L. var. capitata L. (Brassicaceae) and related vegetables in Thailand. Overuse of pesticides on DBM is a serious problem in most vegetable growing areas of the country. Indigenous parasitoids have often been overlooked in the development of IPM programs for crucifer crop pests. Six species of parasitoids were reared from DBM larvae and pupae collected in northern Thailand in 1989-90, 1997-98, and 2003-04. These included the larval parasitoid Cotesia plutellae Kurdjumov (Braconidae), a larval-pupal parasitoid Macromalon orientale Kerrich (Ichneumonidae) and pupal parasitoids Diadromus collaris Gravenhorst (Ichneumonidae) and Brachymeria excarinata Gahan (Chalcididae). Single specimens of Isotima sp. Förster (Ichneumonidae) and Brachymeria lasus Walker (Chalcididae) were also reared from DBM hosts. C. plutellae was the predominant DBM parasitoid and parasitism averaged around 40% from many sites sampled over several years. C. plutellae was reared from host larvae collected not only from abandoned crucifer crops but also from fields sprayed regularly with insecticides. Parasitism by M. orientale was only 0.5-6%. Parasitism of host pupae by D. collaris was 9-31% while B. excarinata pupal parasitism ranged from 9 to 25%. C. plutellae appears to be the most important parasitoid of DBM in Thailand; other native or naturalized parasitoids may play supplementary roles. IPM programs focused on conservation of local DBM parasitoids and on greater implementation of biological control tactics will help alleviate growing public concerns regarding the effects of pesticides on vegetable growers and consumers.