Host-stage selection of Lysiphlebus japonicus Ashmead, a parasitoid of Aphis craccivora Koch and the effect of host stage being parasitized on the parasitoid development and fecundity were investigated at (25[plus or minus]1)[degree]C. When aphids of mixed stages were provided, the parasitoid showed a preference for younger aphids to parasitize. The relative parasitism on the 2nd instar aphids was the highest (26.4%), followed by 1st instar aphids (20.6%), and rather low on the adults. The development of the aphids was delayed after being parasitized. The durations of the 1st, 2nd and the 3rd in-star of the hosts significantly prolonged when 1st instar aphids were parasitized, and those of the 3nd and 4th instar were also markedly extended after alate were parasitized. However, no obvious influence on development of apterae and alate was observed after they were parasitized. The life span of the adults of the aphids parasitized at various nymphal stages sharply shortened. All of the parasitized 1st instar aphids turned to mummies before the adults. The fecundity of the adults from younger aphids parasitized distinctly declined in comparison with the control adults. Immature stage of the parasitoid in 2nd instar of the hosts developed the fastest with duration of 194.10 h, and the slowest of 215.80 h in 1st instar of the hosts. Total contents of protein and glycogen in A. craccivora of different stages were also assayed. Biometrics; Reproduction; Life cycle and development; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Lysiphlebus japonicus (Aphidiidae); Reproductive productivity; Development; Hemipteran hosts; Aphis craccivora; Host stage selection & effect on parasite development & fecundity Aphis craccivora (Aphidoidea); Age; Hymenopteran parasites; Lysiphlebus japonicus; Host stage selection & effect on parasite development & fecundity none