Tritrichomonas fetus causes infertility and abortion in cattle; however, there is scarce information regarding the susceptibility of bovine sperm to this parasite. The objective of this study was to analyze in vitro the interaction between T. fetus and bovine sperm and to evaluate the effect of extracellular products secreted by the parasite on these reproductive cells. Sperm from five fertile bulls (Bos taurus taurus, Holstein-Friesian), selected through a Percoll gradient, adhered to T. fetus after 30 min of interaction, resulting in agglutination between the two kinds of cells. Based on reverse transcription polymerase chain reaction (RT-PCR), T. fetus continuously expressed its gene for cysteine peptidase in the presence or absence of sperm. Computer-assisted semen analysis (CASA) revealed that, after 1 h incubation of sperm in T. fetus culture extract, the extracellular products secreted by the parasite decreased sperm progressive motility (P < 0.05). Although T. fetus extracellular products did not lead to loss of sperm viability (P < 0.05) based on the Annexin-V/propidium iodide assay, the percentage of Annexin-V fluorescein isothiocyanate positive and propidium iodide positive cells increased (P < 0.05) during incubation of sperm in T. fetus culture extract, consistent with cellular damage. In conclusion, extracellular products secreted by T. fetus were cytotoxic to bovine sperm, as they decreased sperm progressive motility; perhaps this contributes to the pathogenesis of T. fetus induced infertility. (C) 2010 Elsevier B.V. All rights reserved.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)