Several ecosystem services directly depend on mutualistic interactions. In species rich communities, these interactions can be studied using network theory. Current knowledge of mutualistic networks is based mainly on binary links; however, little is known about the role played by the weights of the interactions between species. What new information can be extracted by analyzing weighted mutualistic networks? In performing an exhaustive analysis of the topological properties of 29 weighted mutualistic networks, our results show that the generalist species, defined as those with a larger number of interactions in a network, also have the strongest interactions. Though most interactions of generalists are with specialists, the strongest interactions occur between generalists. As a result and by defining binary and weighted clustering coefficients for bipartite networks, we demonstrate that generalists form strongly-interconnected groups of species. The existence of these strong clusters reinforces the idea that generalist species govern the coevolution of the whole community.In the past few years, several studies have used complex network theory to unveil hidden secrets of nature. The patterns of interactions between species provide information about the stability of ecological communities ( One kind of ecological network, mutualistic networks, depicts the interactions of mutual benefit between species in a community. Here, a set of species A, generally animals, interacts with another set of species P, generally plants, but there are no interactions within sets. Such a network can also be referred to as a bipartite network. Species are represented as nodes, and the interactions between these species are represented by links. The information about interactions can be binary, where a link indicates just an interaction between two species, or can be weighted, where the weight represents how strong that interaction is (Bascompte and Jordano 2007).While the aforementioned works have analyzed the structural properties of mutualistic networks in their binary representation, a smaller but growing number of papers have analyzed the relevance of interaction weights. A seminal work by Bascompte and coworkers (Bascompte et al. 2006), found that the asymmetry of dependences (Jordano 1987) between species in an interaction allows for greater biodiversity. Scotti et al. (2007), analyzed how the rank of species importance, based on their position in the network, changes when considering interaction weights. Though the ranks where very different in food webs, they found smaller differences in plant-pollinator networks. Blüthgen et al. (2008), apply information theory measures to characterize the degree of specialization in quantitative mutualistic networks. Hwang et al. (2009), showed that species strength, defined as species' total number of visits, is correlated with species degree in six plant-pollinator networks. Despite these first studies, the complete structure of weighted mutualistic networks, and how interaction weights a...