Abstract: Abstract: MYP (Major yolk protein) gene expression at transcription level in different stages of gonad of sea urchin Strongylocentrotus intermedius and hybrids (S. intermedius♀×S. nudus♂) was analyzed by real-time RT-PCR. Based on normalization with 18S rRNA levels, the comparative quantities of MYP expression were determined. The expression of MYP gene in gonad showed little difference between female and male. MYP gene expression was decreased rapidly in the gonad of S. intermedius at different stages, and slowly in hybrids. The comparative quantities of MYP expression in the gonads of S. intermedius were decreased from 44.55% to 9.59% in female and from 41.17% to 1.83% in male at different stages. The comparative quantities of MYP expression in the gonads of the hybrids were decreased from 37.66% to 19.22% in female and from 36.66% to 12.55% in male at different stages. The results indicated that the difference of MYP expres-sion was correlated with the variation caused by hybridization.