The nuclear DNA content was determined for the first time in 25 species of the South American genus Lessingianthus H.Rob. (Vernonieae, Asteraceae) by flow cytometry. This analysis constitutes the first estimation of the genome size for the Vernonieae tribe. The 2C- and 1Cx-values were calculated in all the species. The 2C-value ranged from 2.04 to 14.34 pg. The 1Cx-value ranged from 0.995 to 1.43 pg. The general tendency indicated a decrease in the 1Cx-value with increasing ploidy level, with some exceptions, in some species the 1Cx-value increased with the ploidy increase. The measuring of DNA content allowed reporting a new cytotype for L. polyphyllus (Sch.Bip.) H.Rob.