Karyotypes of 49 populations belonging to eight species of Mexican Eleocharis (Cyperaceae) are described. Chromosome numbers for Eleocharis densa, E. reznicekii, and E. rostellata are reported for the first time and new numbers are reported for E. macrostachya, E. xyridiformis, and the E. montevidensis complex. Numbers ranged from 2n = 10 to 2n = 60. Dysploidy was the most common mechanism of karyotype variation, which has been detected in four species (E. densa, E. macrostachya, E. reznicekii, and E. xyridiformis). Two species are diploid (Eleocharis parishii and E. cf. montevidensis) and three are polyploid (E. acicularis, E. montevidensis, and E. rostellata). Except for specimens of E. montevidensis complex, no intraspecific variation in chromosome number was found. However, differences in the chromosome sizes were found among populations of that complex and in E. rostellata. Mean lengths of diploid set ranged from 12.96 μm in E. montevidensis to 178.25 μm in E. rostellata and the average of chromosomes sizes varied from 0.97 μm in E. montevidensis to 6.01 μm in E. xyridiformis. These two taxa presented an extreme interchromosomal asymmetry A2: 0.12 and 0.43. Absence of primary constrictions was confirmed. Taxonomical implications of the karyological data are discussed.