The present cytogenetic study performed in argentine populations ofSynbranchus marmoratusshows clear differences between the brazilian samples analized by FORESTIet al.(1992). The results obtained indicate only one chromosomic number from all samples, 2n= 44 (4M-SM + 40 ST-A) and the existence of multiple C-bands either in pericentromeric or interstitial position. Also multiple NORs (on several different chromosome pairs). Were detected the existence of one specimen carrying a small accessory chromosome caracterized as one metacentric and heterochromatic element. Intraindividual polymorphism involving the lenght of pair three was found in all the specimens analized. The comparison between the present data and the available bibliography shows clearly the presence of more than one taxonomic entity withinSynbranchus marmoratus.