none Cotesia (= Apanteles) marginiventris (Cresson), a solitary endo-parasite, was introduced in India for trials against tobacco caterpillar, Spodoptera litura (F.). In an effort to evaluate mass rearing technique, studies on host age preference and biology of this parasite were conducted in laboratory where the temperature was 27 plus or minus 1 degree C and RH 50-60 per cent. For oviposition, 3 to 5 days old larvae were preferred. Mean developmental period from oviposition to cocoon formation lasted 8.2 plus or minus 1 days and pupal period 3 to 4 days. Sex ratio was 1:0.25 (male : female). A single mated female was capable of parasitising on an average 91.3 larvae during its life span. Mean longevity of adult female and male was 5.3 plus or minus 0.81 and 4.0 plus or minus 0.89 days respectively.