Abstract: A survey on the massive death of cultured scallop , Chlamys f arreri was carried out in Jiaozhou Bay in 2000. The environmental factors including salinity, temperature, dissolved oxygen, pH, COD, and NH4+ 2N were investigated , also the pathology of diseased scallop sampled in the period of massive death was studied. The diseasefirst occurred on 6Julyandreacheditspeakstagefrom 21 to 27 July, thensubsidedinthe earlyAugust. The results showed that the cumulative mortality of cultured scallop reached 90. 0 % on 3 August and the monthly highestmortalitywhichnormallyappearedinJulywas 58.0%. Thehighestdailyprevalencewhichoccurredon 24 Julywas 14.5%. Indoorexperimentsprovedthatthe diseasedscallops diedcompletelywithin 2to 3days. The shell height of scallop averaged 5. 0 ±0. 9 cm in the period of massive death. Half of the cultured scallop died on 23 July. The results of investigating and analyzing of the environmental factors , causes of disease and observation of pathology showed that the massive death of scallop was possibly caused by a certain factor of organism such as virus , whereas it has no direct relations with environmental factors.