The morphological characteristics of the dispersal units remain remarkably constant under varying environmental conditions, and may be used as suitable criteria for the taxonomical identification of the species, as well as the whole plants do. This work presents a brief description of the plants and ilustrates and characterizes morphologically the weed seeds of Amaranthus retroflexus L., Chenopodium ambrosioides L., Chenopodium album L., Spergula arvensis L., Stellaría media (L.) Cyrill. e Talinum patens, that will permit a clear taxonomical diferentiation between them. A general description of the order Caryophyllales and a more detailed investigation of the seed of each studied species were carried out, utilizing external and internal morphology. Two different taxonomical keys are presented in order to help in the identification of the seeds and plants. The botanical and common names were included for each species and, whenever possible, a description of the habitat, life cycle, phenology, type of reproduction, geographical distribution, the crop in which it is considered a weed, the agricultural seed in which the dispersal unit is found as a weed seed, and other informations.