The flea beetle Systena silvestrii Bechyné 1957 was studied in context with the evaluation of natural enemies of the alligator weed, Alternanthera philoxeroides (Martius) Grisebach (Amaranthaceae). The female is described and the holotype male is redescribed adding new diagnostic characters: mouthparts, hind wings, metendosternite, male and female genitalia. Differences in color patterns between S. silvestrii, S. marcapatensis Bechyné, S. scurra scurra Scherer and S. scurra ajonjoli Bechyné Bechyné are considered. This flea beetle was collected mostly on alligator weed but also on Phyla canescens (Vebenaceae), in Argentina, Uruguay, Paraguay and Brazil. Therefore, further evaluation is strongly recommended prior to its consideration for biocontrol of A. philoxeroides.