Echinochloa helodes (Hackel) Parodi grows naturally in Argentina, Uruguay and Brazil. It is a perennial, semihalophyte, rhizomatous and hygrophylic Summer grass, in naturally wet lands it is a good forage resource. This paper analyzes numeric chromosomic variants, sexual reproduction and seed retention in four populations of Argentina and Uruguay, through the study of meiotic behavior, pollen stainability and seed production in self-and open-pollinated panicles. All collections showed 6x level, with 27 bivalents at diakinesis-metaphase I and normal migration 27 : 27 in early anaphase I. Pollen stainability was 93%. Seed set was poor (10.5%) in self-pollinated panicles, but over 53% after open pollination, indicating the prevalence of allogamy. No seed retention was found.