Compsilura concinnata is a gregarious tachinid which lays eggs directly into a host larva with a sickleshaped ovipositor. The oviposition and immature development of C. concinnata was examined using the noctuid moth Mythimna separata as a host. C. concinnata females usually laid one egg per single ovipositional attack, but occasionally laid more than one egg at a time. Also, the females often deposited no eggs in both moving and non-moving hosts, suggesting that this imperfect oviposition was not caused by the defense behaviour of the host. Successful development from egg to adult was obtained within a temperature range of 15.0-27.5°C. At 30°C, this fly could pupariate but failed to emerge as an adult. The developmental time from oviposition to adult emergence decreased with rising temperature. The lower developmental threshold from oviposition to adult emergence was 7.7°C for females and 8.7°C for males. The total effective temperature from oviposition to adult emergence was 404.4 degree-days for females and 342.1 degree-days for males.