Abstract: Viruses are recognized as ubiquitous and numerically abundant components of marine ecosystems. To study the spatiotemporal distribution and seasonal variation of virioplankton and bacterioplankton, the relationship among virioplankton, bacterioplankton and Chl-a in the coastal waters of Jiangsu, four cruises of oceanographic survey were conducted in July and December, 2006 and April and October 2007. Virioplankton and bacterioplankton were stained with SYBR Green I and analysised by epifluorescence microscopy, and Chl-a was analysised by TD-700 fluorescence analyser. The highest virioplankton abundance was 47.90?06/mL occurred along shoal of north of Jiangsu, the lowest was 0.03?06/mL occurred in L黶i waters, and the highest value was more 1000 times than the lowest. The average abundance of virioplankton was the highest in winter, higher in summer, and the lowest in spring and autumn. Horizontal distribution showed the virioplankton abundance was higher in the central section of coastal of Jiangsu than in two sides. Meanwhile, it got decreasing along the vertical shore direction. Vertical distribution also showed temporal variations, and there were higher abundance in surface water than in bottom in spring, summer and autumn. The virus-bacteria abundance ratio (VBR) was 0.30-180.08, on average was 18.35, the highest VBR occurred in winter, and the lowest in spring. Seasonally, the average VBR was summer (28.82) winter (20.41) autumn (12.89) spring (11.26). VBR was explained the relationship between viruses and prokaryote cells (bacteria). The coefficient between VBR and bacterioplankton was negative correlation, and the correlation coefficient was -0.19, -0.42 and -0.39 (P0.05) in spring, summer and autumn. However, there was no clear correlation in winter. Otherwise, the correlation between VBR and bacterioplankton could not be indicator as hosts bacterial diversity in Jiangsu coastal area, although it was believed to be common in open sea. Strong relationship existed between virioplankton and Chl-a, virioplankton and bacterioplankton in spring, and the correlation coefficient was 0.79 and 0.74 (P0.01), respectively, and only obvious correlation between virioplankton and bacterioplankton occurred in autumn, the correlation coefficient was 0.79 (P0.01). However, there were no marked correlation in summer and winter among them. These results implied that there were different hosts for marine viruses to infect, and the virus-bacteria population dynamics seemed to be complex in different seasons. In one word, the results suggested that marine viruses were import components in the coastal waters of Jiangsu.