The objectives of this study were to determine the average dimensions, to describe the external structure of seeds, fruits or spikelets and to estimate the number produced per plant and per gram of twelve weed species. Propagules sketchs were elaborated. The number of seeds per plant varied from 1290 in Asclepias curassavica to 195.008 in Ludwigia sp within the dycotyledoneae. The second specie with the highest average number of seeds per plant was Sida rhombifolia (7962) followed by Ipomoea sp.(2876). Hyptis capitata showed a higher number of heads per plant (341) than Bidens pilosa (106). Mimosa pudica produced an average of 3.3 seeds per fruit. In relation to Poaceae species Paspalum conjugatum y P. Virgatum produced a larger number of spikelets per floral branch (381 y 1185 respectively) than Ischaemum indicum (81) and Rottboellia cochinchinensis (14). Anemocory, hydrocory and zoocory are mentioned as mechanisms contributing to dispersal of these species.