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Abstract: The morphology and morphometric data of seven populations of Blepharisma sinuosum from southeastern Brazil were investigated. The description is based on live observations, protargol impregnation, and scanning electron microscopy. Blepharisma sinuosum measures 75–255μm in length and 25–93μm in width and has a spindle-shaped body, pink color, a single contractile vacuole located at the posterior end, 50 adoral membranelles, a conspicuous paroral, 17–35 somatic kineties, a moniliform macronucleus with 2–7 connected nodules, and 3–20 micronuclei. Morphological comparisons with similar species were performed and suggest that B. americanum is the junior synonym of B. sinuosum. The 18S rDNA gene sequence of B. sinuosum was obtained and compared with that of other Blepharisma species. The length and GC content of the obtained sequence is 1652bp and 47.03%, respectively, and has a very high structural similarity (99.9%) with the B. undulans sequence. The validity of the classification of Blepharisma species in morphonuclear subgenera is also discussed. [Copyright &y& Elsevier]