Seed mass and shape of grasses were analysed in a temperate flora containing 178 and 26 species belonging to C3 and C4 photosynthetic types respectively. The weedy character and the annual or perennial status were also considered. On the basis of the seed traits studied, three groups were distinguished: C4 grasses, annual C3 grasses of weedy character and perennial non-weedy C3 grasses. The C4 group had more isodiametric (same diameter in all directions) seed shape and lower average seed mass than the C3 group. To our knowledge, this has not yet been described for temperate C4 grasses and is certainly associated with their preference for open habitats where competition for light is small. Weedy annual C3 grasses had heavier and less isodiametric seeds than C4 grasses did. These species are mostly specialized to establish in the dense cover of perennial vegetation, and this ability distinguishes them from the C4 group. Non-weedy perennial C3 species possessed less isodiametric seeds than did C4 grasses, but did not differ from weedy annual C3 grasses. As most alien C4 grasses naturalized in Hungary are annuals with small, isodiametric seeds, these traits are good candidates to be included in screening for potential future invasives in open habitats.