To examine the effect of different two-carp ratios between silver carp Hypophthalmichthys molitrix and bighead carp Aristichthys nobilis cocultured in the mussel pond on growth of freshwater pearl mussel Hyriopsis cumingii and water quality,an enclosure experiment was carried out in Tangxi of Zhejiang Province from April 23 to September 21,2008.Two-carp ratio was set at six levels:0/0(control group),100/0,70/30,50/50,30/70 and 0/100.Shell length,shell width and wet weight of pearl mussels were measured at the beginning and the end of the experiment,respectively; and the water quality parameters including NO₃-N,NO₂-N,NH₃-N,TN,TP,PO₄-P,COD were determined twice a month during the experiment.The results showed that the relative growth rate of mussel shell length in level 100/0(silver carp vs.bighead carp ratio)were significantly lower than that of levels 0/0,50/50 and 0/100(P<0.05),and no significant differences were found among survival rate,growth rate of shell width and wet weight of mussels at various polyculturing two-carp ratio levels(P>0.05).Besides,enclosures at level of 30/70 had better water quality in which TP was significantly lower than 100/0(P<0.05),COD was significantly lower than 100/0 and 70/30(P<0.05),NH₃-N was significantly lower than 100/0(P<0.05)and PO₄-P was significantly lower than 70/30(P<0.05).So the ratio of 30/70 of silver carp vs.the bighead carp was the best polyculture ratio for mussel culture in terms of the growth and water quality parameters.