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A taxonomic study of the genus Strombomonas Defl. was carried out in three reaches of the stream Bom Jardim, Municipality of Triunfo, Rio Grande do Sul State, expanding the knowledge of its richness and geographical distribution. Samples were collected monthly (2002-2006), with plankton net and preserved with formaldehyde. Simultaneously, water samples for measurement of physicochemical variables were collected. Twenty-six species of Strombomonas were identified; eight taxa have cosmopolitan distribution and the remainder are restricted to a maximum of three continents. The cluster analysis indicated that the greatest similarity occurred between the stream sections T2 and T3, equal to 82%, differently from T1 (upper reaches). This difference can be explained considering that in T1 the exclusive presence of the species S. brevicaudata,S. scabra var. labiata, and S. rotunda was verified. Strombomonas brevicaudata Conf. is registered for the first time in the State. Strombomonas eurystoma (Stein) f. incurva (Buz.) Pop., and S. recurvata Conf. & Joo are new records for Brazil.

Foram identificados 34 táxons específicos e infra-específicos do gênero Strombomonas Defl. resultado do estudo de amostras qualitativas no Parque Estadual Delta do Jacuí, localizado a 29°56'-30°03'S e 51°12'-51°18'W. As amostragens foram realizadas em 25 ambientes aquáticos em abril/1993 e janeiro/1994 e maio/1998 a setembro/1999. Strombomonas borystheniensis, S. ensifera, S. gibberosa, S. girardiana, S. scabra, S. triquetra var. torta e S. verrucosa foram os táxons mais freqüentes na área, ocorrendo em mais de 50% dos locais amostrados, destacando-se S. verrucosa pela ampla distribuição na área do Parque, com a ocorrência em 96% dos locais estudados. Strombomonas chodatti, S. cuneata, S. moreniensis,S. napiformis var. brevicollis são primeiras citações de ocorrência para o Estado e Brasil. S. confortii,S. fluviatilis var. major, S. morenensis, S. scabra var. intermedia e S. scabra var. ovata f. minor ocorrem exclusivamente na América do Sul. São realizadas considerações taxonômicas e ecológicas sobre o gênero, assim como é fornecida sua distribuição nacional e mundial. Thirty four specific and infra-specific taxa of the genus Strombomonas Defl. were identified as the result of the study of qualitative samples from Jacuí Delta State Park, that is located for 29°56'- 30°03'S and 51°12'-51°18'W. Sampling were carried out in 25 stations from April/1993 to January/1994 and May/1998 to September/1999. Strombomonas borystheniensis, S. ensifera, S. gibberosa, S. girardiana, S. scabra, S. triquetra var. torta and S. verrucosa were the most frequent taxa in the area, occuring in more than 50% of the samples, being distinguished S. verrucosa for its wide distribution in the area of the Park, with the occurrence in 96% of the studied places. Strombomonas chodatti, S. cuneata, S. moreniensis and S. napiformis var. brevicollis,were registered for the first time to the State and Brazilian territory. S. confortii, S. fluviatilis var. major, S. morenensis, S. scabra var. intermedia and S. scabra var. ovata f. minor are taxa that occur exclusively in South America. Taxonomic and ecological considerations are supplied as well as its national and world distribution.