A preliminary taxonomical survey was carried of the thecate Dinophyceae based on samples collected in July 1983 and November 1984 from Barra do Bugres, Cáceres, Chapada dos Guimarães, Porto Esperidião and Quatro Marcos Municipalities, State of Mato Grosso, Brazil. Three taxa belonging to the genus Peridinium Ehr. and one species of Sphaerodinium Wol. were identified. All species are new records for the Mato Grosso State and one species (P. africanum Lemm.) for Brazil.