Toxic effects of diuron and carbofuran on Paramecium caudatum were evaluated. Acute and chronic tests were conducted with diuron and carbofuran active ingredients and their commercial formulations, Diuron Nortox® 500 SC and Furadan® 350 SC, respectively. The sensitivity range of P. caudatum to reference substance sodium chloride was established. A preliminary risk assessment of diuron and carbofuran for Brazilian water bodies was performed. The tests indicated that toxicity of pure diuron and its commercial formulation was similar, while the commercial product carbofuran was more toxic than its pure form. In acute tests, readings were carried out at 2, 3, 4 and 6 h and showed an increase of mortality with increasing exposure time. The sensitivity of P. caudatum to NaCl ranged from 3.31 to 4.44 g L−1, averaging 3.88 g L−1. For diuron, the 6 h LC50 was 64.6 ± 3.3 mg L−1 for its pure form and 62.4 ± 2.5 mg L−1 for its commercial formulation. Carbofuran active ingredient was less toxic than that of diuron, presenting a 6 h LC50 of 142.0 ± 2.4 mg L−1 for its pure form and 70.4 ± 2.2 mg L−1 for its commercial product. Chronic tests showed that these pesticides cause significant decrease on population growth, generation number and biomass of P. caudatum. The 24 h IC50 was 7.10 ± 0.58 mg L−1 for pure diuron, 6.78 ± 0.92 mg L−1 for commercial diuron, 22.95 ± 3.57 mg L−1 for pure carbofuran and 4.98 ± 0.62 mg L−1 for commercial carbofuran. Preliminary risk assessment indicated that diuron and carbofuran present potential ecological risks for Brazilian water bodies. P. caudatum was a suitable and sensitive test organism to evaluate diuron and carbofuran toxicity to freshwater protozooplankton and, taking into account the relevant role of protozoans in aquatic environments, we strongly recommend its inclusion in ecotoxicological studies. •Diuron and carbofuran decreased growth, generation number and biomass of P. caudatum.•The toxicity of pure and commercial form of diuron was similar.•Commercial product carbofuran was more toxic than its pure form to this protozoan.•P. caudatum is a suitable test organism to evaluate pesticide toxicity.•Diuron and carbofuran represent potential ecological risks for Brazilian water bodies. Diuron and carbofuran chronic exposure cause significant decrease on population growth, generation number and biomass of Paramecium caudatum.