The extracts of brain of Macrobrachium rosenbergii were applied on a Sephadex G -50 column (1. 6cm ×50cm) . Two large peaks of protein , peak Ⅰand peak Ⅱ, were obtained. The ⅡC(63 -74 fractions) of peak Ⅱwere able to increase oocyte diameter of M . rosenbergii in vitro very significantly. The 63 -65 fractions from ⅡC of peak Ⅱ significantly accelerated the total protein synthesis of the prawn ovary in vitro. By injecting the 63 -65 fractions into muscle of M . nipponense the ovary maturation was promoted significantly after 15 days. The molecular weight of the protein in the fraction 63 was below 7kD. The results show that the gonad2 stimulating hormone is present in the brain of M . rosenbergii.