Abstract: Knowledge about the breeding systems of invasive weeds is crucial for invasion management. Eichhornia crassipes, a noxious clonal plant, has a complex breeding system that includes clonal reproduction and a special sexual reproduction mode, tristyly, and a complex mating system that consists of three floral morphs that differ in the length of styly and stamens. In this paper, we compared the breeding systems of 40 Eichhornia crassipes populations in 24 localities throughout China by investigating the relative level of sexual and clonal reproduction, number of floral morphs and the variances in floral syndrome (especially the position of sexual organs) for each population. A Mann-Whitney test was used to detect whether the variation in the position of the sexual organs were significant among each other and the typical floral morphs. The seed-set ratio of each floral morph was investigated extensively in a Chongqing population that had a high number of floral morphs. All the populations were regenerated primarily by clonal propagation as a result of vigorous clonal growth. Sexual reproduction was presumably constrained because only one or two floral morphs existed in the populations. Sexual recruitment was further restricted by lack of suitable ecological conditions for seed germination and seedling establishment. Exposure to the sunlight and water temperature above 30 ℃ is generally necessary for seed germination, a condition not met in most populations. Most of the populations surveyed were monomorphic for the M floral morph, and only the populations in Chongqing (West China on the Yangtze River) and Nanning (Southwest China near the tropical zone) contained the L morph with the M morph still predominating the populations. No significant differences in the fecundity of the M and L morphs were found and only 4% of the individuals produced seeds for these two floral morphs. The number of seeds per capsule varied greatly in both morphs, from 13 to 296 in the L morph and from 20 to 230 in the M morph, which suggested that fecundity was greatly restricted by ecological conditions, such as pollination and weather conditions, rather than developmental deficiency. The variations in the position of the sexual organs in the M floral morph were not significantly different among each other and the typical flowers. But, populations in Chongqing, Nanning, Fuzhou and Haikou were found to have modified M morphs or L morphs with anthers and stigma adjacent to each other (namely semi-homostylous flowers). Semi-homostyly is thought to be controlled by modifier genes and is selected for since it can increase fecundity through autogamy in invaded regions that have unreliable and inefficient pollinators. Based on the relative levels of sexual reproduction and number of floral morphs in a population, our results showed that the breeding systems of populations in the southwestern region and on Hainan Island were much more diverse than in other regions. The evolutionary trend of the breeding systems that occurred in some E. crassipes populations is consistent with the Baker's law, which states that autogamy would be favored during an invasion event. These evolutionary changes of the breeding systems also have significant effects on the invasion process of this species.