The larval parasitoid Microplitis manilae Ashmead (Hymenoptera: Braconidae) is a potential biological control agent of Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae). To understand the preference and fitness of M. manilae on larval instars of S. exigua, we compared host choice, development, and life table parameters when different larval instars of S. exigua were supplied in the laboratory. Results showed that parasitism of 2nd or 3rd instar larvae was significantly higher compared with other instars. The intrinsic rate of increase (r), finite rate of increase (λ), net reproduction rate (R₀) and mean length of a generation (T) were significantly affected by which larval instars were attacked. The maximum values of r, λ, R₀ and T were observed when M. manilae parasitized 2nd instar S. exigua larvae. Therefore, we conclude that the 2nd larval instar of S. exigua represents the optimum host stage and suggest that 2nd larval instar of S. exigua will be the most suitable host stage for mass production of M. manilae as well as the best instar to target for biological control in the field.