The floristic inventory of Thalassiosirales from Guaraguaçu river was based on planktonic and periphytic samples collected at six sampling stations throughout the river, considering a salinity gradient. A total of 28 species were identified with 19 included in Thalassiosiraceae family. Among the diatoms recorded, 85,7% were marine/estuarine and 14,3% were continental species. Cyclotella atomus Hustedt, C. choctawhatcheeana Prasad, Thalassiocyclus lucens (Hustedt) Hakansson & Mahood, Thalassiosira minima Gaarder, T. prochkinae Makarova, T. pseudonana Hasle & Heimdal, T. tealata Takano, T. tenera Proschkina-Lavrenko and Thalassiosira sp. were the nine new records for the State of the Paraná. Identification keys, taxonomic descriptions, measurements, MO and MEV illustrations complement the study.
lotic system; Coscinodiscophyceae; diatoms; estuary; taxonomy
