none The effects of strains of Plutella xylostella (Lepidoptera: Plutellidae), strains of Cotesia plutellae (Hymenoptera: Braconidae), fenvalerate and their interactions on the biological characteristics of C. plutellae were studied in the laboratory. Two strains of P. xylostella, one susceptible to fenvalerate (SP) and the other resistant (RP), and two strains of C. plutellae, one that had been reared using SP as host (SC) and the other which had been reared using RP as host (RC), were arranged in four host-parasitoid combinations, i. e ., SC-SP, SC-RP, RC-SP, and RC-RP. The results showed that, in the absence of the insecticide, the parasitoids in the SC-RP combination had a lower percentage of cocoon formation (45.8%), shorter cocoon length (7.8 mm) and females had shorter forewings (3.28 mm) and hind tibia (2.33 mm), compared to those in SC-SP, RC-SP, and RC-RP combinations. This indicated that insecticide resistance in the host had detrimental effects on the parasitoid. When fenvalerate was applied to the host-parasitoid system, percentages of cocoon formation in the RC-RP and SC-RP combinations were 95.5% and 37.8%, respectively, significantly higher than those in the SC-SP (22.5 %) and RC-SP (25.8 %) combinations, suggesting that host insecticide resistance offered some protection to the parasitoid larvae inside the hosts. When the RC-SP combination was either unexposed or exposed to the insecticide, the parasitoids had similar percentages of adult emergence (93.6% vs 95.2% ) as well as similar development times, indicating that the parasitoids, which had been reared on a resistant host strain, still showed some tolerance to the insecticide when parasitizing a susceptible host strain, and this effect should favour the development of insecticide resistance in the parasitoid. This demonstrates co-evolution in the development of insecticide resistance in this host-parasitoid system. Biometrics; General morphology; Appendages; Life cycle and development; Development; Metamorphosis; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Animal constructions; Pollution; Chemical pollution; Abiotic factors; Chemical factors Plutella xylostella (Plutellidae); Hymenopteran parasites; Cotesia plutellae; Parasite biology, insecticide resistance & insecticide resistance significance; Fertilizer and pesticide pollution; Fertilizers and pesticides; Fenvalerate Cotesia plutellae (Braconidae); Size; Cocoon & forewing length; Thoracic legs; Tibia length; Wings; Forewing length; Emergence; Emergence rate; Lepidopteran hosts; Plutella xylostella; Cocoons; Fertilizer and pesticide pollution; Biology effects on resistant vs susceptible strains in resistant vs susceptible lepidopteran hosts; Fertilizers and pesticides; Fenvalerate