There is increasing attention being given to the importance of colonization traits like seed number and seed size for species coexistence and distribution in plant communities. In this study a null-model analyses of species co-occurrence and trait dispersion was performed to evaluate the importance of seed size and seed number for coexistence in a Swedish semi-natural grassland. Species abundance was inventoried in two grids based on quadrats differing in size (1 dm2 and 25 dm2). For both grids Monte Carlo simulations showed that species (in general) co-occurred more often than expected from a null model. Both seed size and seed number were underdispersed, species possessing similar seed traits tended to co-occur more often than expected from a random model. Species packing in a niche space based on seed number and seed size was found to be positively correlated to species richness. In species-rich sites species were more densely packed. The results contradict predictions from classical competition theories concerning species coexistence, and suggest that seed traits are important for species coexistence.