Abstract: Tetraploidy was induced by inhibit ing the f irst polar body in eggs from triploid Pinctada martensii ( Dunker) fertilized with haploid sperms with 0. 5..g/ mL cytochalasin B ( CB) treatment. The effects of treatment duration and timing of treatment after fertilization on embryonic hatching rate and the percentage of tetraploid were examined, the growth and survival of larvae were compared between treated group, untreated group and diploid control. The results indicated that the hatching rate was inversely related to treatment duration time, but the percentage of tetraploid was positively related to treatment duration time. Two optimal moments for treatment application were determined. Optimal treatment duration time was 15..18min, optimal t iming of treatment was 3..5min before the release of polar body 1. The percentage of tetraploidy embryos was 20% averagely, the percent harvest of spat was only 0. 02% due to a heavy mortality of larvae. There was no difference in growth of larvae between treated group and untreated group( p > 0. 05) , but the growth of larvae in both groups was slower than that of diplo id control and the difference was significant ( p< 0. 05) .