A taxonomical study about the genus Strombomonas Deflandre from Yacarey stream (Corrientes, Argentina) was carried out. Samples were collected monthly between February
1996 and January 1997. A total of 29 taxa were recorded, among these, S. acuminata var. deflandreana Conr., S. spiralis (Playf.) Defl., S. subcurvata (Proschkina-Lawrenko) and S. vermonti (Defl.) Defl. var. vermonti f. vermonti were recorded for the first time in South America. Additionaly, these S. balvayi Bourr. & Couté, S. fluviatilis var. elegans Drez., S. fluviatilis var. major Conforti, S. planctonica (Wol.) Popova and S. praeliaris (Palmer) Defl. were recorded for the first time in Argentina. Strombomonas girardiana (Playf.) Defl., S. costata Defl., S. verrucosa (Daday) var. zmiewika (Swir.) Defl., and S. triquetra (Playf.) Defl. var. torta Rino were the most frequent taxa, being found in more than 70% of the samples.
