Laboratory studies were conducted to elucidate biological features of Microplitis plutellae (Muesebeck), a larval parasitoid of diamondback moth, Plutella xylostella (L.). M. plutellae was introduced from the United States to Taiwan in 1955 to assist the local braconid, including Cotesia plutellae to control Plutella xylostella. Normally M. plutellae is a solitary parasitoid laying one egg per host larva; however, at times, more than one egg were found in the same host. Although early instar larvae were present in the host larval hemocoel, only one parasitoid survived to come out of host larva for pupation, killing the larva in the process. M. plutellae had three larval instars, each with unique morphological characteristics. The larval period was 7-10 days at 25-30oC. Mature larva exited the host body and spun a spindle-shaped brown cocoon from which an adult emerged after 5-6 days of the pupation period. Majority of the adults emerged during 0300-0900 hours. Oviposition largely took place during the daylight hours. Adult longevity increased up to 16 days when provided with food, honey solution; however, it lasted less than 2 days in the absence of food. The total developmental time from egg to adult was 16-18 days at 25-30[degree]C. Plutella xylostella (Plutellidae) : Host Microplitis plutellae (Braconidae) : Parasite General morphology; Reproduction; Reproductive behaviour; Reproductive productivity; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Microplitis plutellae; General morphology; Egg laying; Oviposition; Fecundity; Life cycle and development; Lepidopteran hosts; Biology; Plutella xylostella Plutella xylostella; Hymenopteran parasites; Parasite biology; Microplitis plutellae none