Abstract: n the paper, the quantitative variation and distribution of microorganism population in shellfish culture shoal of south bank in Hangzhou Bay was studied, The samples were collected from 3 different sites in Hangzhou Bay. The results indicate that heterotrophic bacteria variation is from 1.62?10^3-6.62?10^5cfu?g^-1, mean 4.28?10^4cfu?g^-1, and there is a low quantity in spring and summer, rising in October and reaching the peak in November in the sample sites. About bacteria populations, dominant species is gram-positive bacteria, which is characterized with an obvious origin from land. Denitrifying bacteria, ammonifying bacteria and sulfate-reduction bacteria all were found in these sites and maintain a high level although there was a difference in quantity between bottom and surface layers in the shoal, surface layer being higher than bottom.