none The effectivity of various parasites on the immature stages of the green bean looper, R. nu Guenee, was elevaluated through periodic samples conducted during 2 seasons (1981 and 1982) taken from a pesticide free alfalfa field in Santiago. Eggs were parasitized by Prospaltella porteri Mercet (Aphelinidae) and Trichogramma sp. (Trichogrammatidae) being the former species more effective than the latter. Parasitism assessment upon 4 larval instars demonstrated the occurrence of 4 parasites i.e. the tachinids Incamyia chilensis Aldrich and Voria ruralis Fallen, the braconid Rogas nigriceps Brethes and the ichneumonid Campoletis sp. The most effective larval parasites were I. chilensis and R. nigriceps. The tachinid acted preferently upon medium and large sized larvae, the Hymenoptera preferred small and medium size host larvae. Only 1 pupal parasite was found, the ichneumonid Coccygomimus fuscipes Brulle, a species not previously reported on this host. Its control was effective since it gradually improves as the season proceeds. In adition to the natural mortality of immature stage of Rachiplusia nu due to the global action of the reported parasites, it was determined that the looper populations are maintained below its economic threshold by the combination action of pathogens and other mortality factors.