none The internal parasite Microplitis rufiventris Kok. passes through 3 instars but moults 3 times within its host (Spodoptera littoralis). The last moult occuring just at emergence time. The morphology of the egg and larval stages of the parasite are discussed. At 37 degree C and a photoperiod of 6 h (6L:18D) the endo-developmental cycle of the parasite can summarize as follows: Egg 18-24 h; instar 1,4 days (fighting phase 48 h; feeding phase 30-48 h); instar 2, 12-18 h and instar 3,3 days. The effect of different photoperiods on the relative speeds of the endo-developmental stages of the parasite at each of 30, 25, 20 degree C were carefully studied. At the first 2 temperatures, the short photoperiod (6L:18D) accelerated the development of larval instars, while both of 18L:6D or 0L:24D slowed down the development. Under the latter photoperiods some larvae failed to moult and had emergence problems. The influence of photoperiod is significantly noticeable at 20 degree C. The incubation period of the egg-stage was prolonged significantly at 18L:6D and the development of larval instars was significantly faster and refined at 6L:18D. The factor(s) inhibiting the development of the egg-stage perhaps differ from those affecting the larval development. The ventral area of the host mid-gut among malpighian tubes seems to be where the surplus parasite larvae are eliminated by physical attack. A physiologically suppressed parasite larva is able to attack its developed competitor of the same age. Teratocytes cells perhaps play a part in eliminating the surplus parasite larvae by physiological suppression.