Abstract: An investigation was made on the abundance distribution and its variation of virioplankton in the Yangtze River estuary and its adjacent East China Sea in summer 2006 and winter 2007. The virus direct count (VDC) was quantified by fluorescent microscopy. In summer 2006, the VDC was from 2.22×10⁶ to 9.97×10⁷ ind·ml^-1, being higher in the surface sea water of B area (122.5°-123.5° E); in winter 2007, the VDC was from 1.99×10⁶ to 2.66×10⁷ ind·ml^-1, being higher in coastal A area (120.5°-122.5° E) and decreased from nearshore waters to offshore waters. In summer, the VDC had close relationships with bacteriaplankton biomass and chlorophyll-a concentration but less relationship with nutrients concentration (P>0.05); in winter, the VDC was closely related to bacteriaplankton biomass and nutrients concentration but less related to chlorophyll-a concentration (P>0.05). The abundance distribution pattern of virioplankton showed significant seasonal variability, with the VDC significantly larger in summer than in winter (P<0.01), which was mainly decided by the distribution of the viral host, and partly by the nutrients concentration in wintertime.

Key words: virioplankton, virus direct count (VDC), Yangtze River estuary, summer, winter