An eight-year-old thoroughbred mare with no previous history of illness aborted a fetus at 196 days of gestation, and its internal tissues were examined immunohistologically and bacteriologically. The placenta was not examined, but specimens of the intrauterine fluids and the dam's faeces were collected four days after the abortion and examined bacteriologically. No significant histological lesions were found in the fetus but the amnion and the umbilical cord were oedematous and had petechial haemorrhages. Rhodococcus equi was isolated in pure culture from the lung, heart and stomach contents of the fetus and from an intrauterine specimen and faeces of the dam. The anti-R equi antibody titre of the mare was high after the abortion. The diagnosis was confirmed in the lung of the fetus by immunohistochemical staining with R equi-specific antibodies. Isolates from the fetus and mare were identified as avirulent R equi by pcr and the mouse pathogenicity test. The avirulent isolates were characterised by pulsed-field gel electrophoresis, which yielded only one VspI profile in all the isolates from the fetus and its dam.