We describe the existence of two Echinodorus tennellus cytotypes, with diploid populations, of 2n=22 chromosomes, and triploid ones, with 2n=33. Meiotic studies showed abnormalities and 50% pollen stainability in plants of triploid populations, but 100% pollen stainability in a diploid population. Data from C-banding and CMA/DAPI (Chromomycin A 3 /4,6-diamidino-2-phenylindole) fluorescent banding showed the diploid population has one CMA+/DAPI+ band, equivalent to the heterochromatic region of C-banding, while a triploid population has two CMA+/DAPI+ heterochromatic regions, suggesting an autopoliploid origin for the triploid cytotype. The autotriploid origin was also supported by the trivalent pairing of chromosomes and the presence of three sets of each chromosome in mitotic metaphase. Diploid and triploid cytotypes were easily maintained through vegetative propagation of this stoloniferous species.