Weed infestation is a major threat to rice production, and herbicides have been used extensively to control weeds in paddy fields. However, increasing environmental pressures against herbicide use have led farmers to consider alternative approaches to weed control and more emphases have been placed on the development of ecologically sound weed management strategies. Ducks can be used to control weeds, and rice-duck farming systems, which integrate raising ducks into rice cultivation methods, provide an innovative approach to weed management. Field studies were conducted during 2000-2003 in Danyang, Jiangsu Province, to evaluate the long-term influence of rice-duck farming systems on the structure and dynamics of weed communities in paddy fields, including effects on the population density, species richness, species diversity, community evenness and similarity of weed communities across years. The results showed that under long-term rice-duck farming systems, the density of paddy weeds decreased annually and the trend was best fit by the model, y=k+a·e^bx, where the parameter b indicated the rate of decrease of different weed populations. Of the six main weed species in paddy fields, the population density of Fimbristylis miliaceae, Lindernia procumbens, and Ludwigia prostrata decreased rapidly, followed by slower decreases in Monochoria vaginalis, Cyperus difformis, and Echinochloa crusgalli. Rice-duck farming decreased the species richness and species diversity in paddy fields, and increased the evenness of weed communities. Compared to weed communities before the introduction of ducks, Sorensen's similarity indices of weed communities in paddy fields decreased every year, revealing that the structure of weed community was changed substantially. Rice-duck farming provided a more diversified environment for weed growth and seed storage, which greatly limited weed infestation in paddy fields. Consecutive surveys showed that rice-duck farming was an effective control of paddy weeds with more than a 99% overall reduction by the end of the fourth year. Rice-duck farming is an effective and ecologically-based weed management strategy that has significant economic and ecological benefits.