The biology of the immature stage of Neochrysocharis formosa, a larval idiobiont-endoparasitoid of agromyzid leaf miners, was studied to gain a better understanding of N. formosa and in vitro rearing. Eggs have a smooth surface with a thin, transparent chorion. The length and width are about 190 μm and 60 μm, respectively. Eggs hatched in an average of 32.6 h. Three larval stages are decided by observation of the pre-ecdysis phase. Larvae are hymenopteriform with small mandibles and have 13 body segments. Larval duration at each instar is as follows: 1st instar 35.1 h, 2nd instar 35.7 h and 3rd instar 46.5 h. Mature 3rd-instar larvae escaped from the host and pupated within the mine of the host larvae. Pupal duration was 8 days. The host larvae stopped moving a few minutes after parasitoid oviposition. To examine the oviposition site, longitudinal sections of parasitized host larvae showed all eggs to be within the host’s hemocoel. Fewer eggs were oviposited in the anterior part than the middle and posterior part of the host larvae. No encapsulation or melanization were observed around the parasitoid eggs.