Abstract: Fertilizing rates were determined at various developmental stages of rainbow trout embryos by fluid J1, and fluid J2 and fixed by Bouin＇s fluid and then stripped membrane. The fertilizing rates of the embryos treated by differential fluid J1 were found to be 53.21% at two cell stage, 77.89% at four cell stage, 82% at eight cell stage, 88.2% at morula stage, 87.71% at early blastula stage, and 81.43% at mid-gastrula stage. The embryos treated by differential fluid J2 had fertilizing rates of 51.64% at two cell, 77.86% at four cell stage, 81.43% at eight cell stage, 89.07% at morula stage, 87.50%, at early blastula stage, and 81,86% at mid-gastrula stage. The results suggested that the two kinds of differential fluid be all used detection of fertilizing rate in production practice without sig- nificant differences from the Bouin＇s fluid treatment （P 〉0.05 ）. There was different distinguishing accuracy at various developmental stages, far lower at cell division stage than from morula stage to blastula stage, which is attributed to the asynchrony of embryonic de- velopment and small amount of embryonic cells in prophase. The optimal developmental stage for detection of fertilizing rate of rain- bow trout was observed from morula to blastula （ 12-24℃·d）.

Key words: Oncorhynchus mykiss, fertilizing rate, egg membrane treatment, embryonic development