Trichomonosis is a venereal disease of cattle caused by Tritrichomonas foetus, a flagellate protozoan parasite of the genus Tritrichomonas in the family Trichomonadidae. Trichomonosis is asymptomatic in bulls, however in cows the disease is characterised by infertility, abortion, embryonic and early fetal death, fetal maceration, pyometra and vaginal discharge. The disease has a world-wide distribution and, at one time, was of major economic importance as a cause of abortion and infertility, especially in dairy cattle. The widespread use of artificial insemination in many areas of the world has contributed to reduced prevalence. Tritrichomonas foetus has been reported in domestic cats, horses and roe deer. Other species, such as goats, pigs, dogs, rabbits and guinea-pigs, have been experimentally infected. Tritrichomonas foetus has also been isolated from cats with diarrhoea and is now commonly known as the ‘cat genotype’ T. foetus. Transmission of the disease is primarily by coitus, but mechanical transmission by insemination instruments or by gynaecological examination can occur. The organism can survive in whole or diluted semen at 5°C. Bulls over 3–4 years old are the main reservoir of the parasite as they tend to be long-term carriers, whereas most cows and young bulls (less than 3 years old) may clear the infection spontaneously. A partially efficacious, killed whole-cell vaccine is commercially available. Tritrichomonas foetus has also been reported to cause infections in humans, including meningoencephalitis and peritonitis in immunocompromised and immunosuppressed individuals.