Introduction: The majority of abortions in mares are associated with placental infections caused by opportunistic bacteria. Rhodococcus equi is widespread in the environment of stud farms and Klebsiella oxytoca is found on the mucosal surfaces of horses. Both can occasionally cause placental infection and abortion in mares. To the best of our knowledge, however, there have been no reports of R. equi and K. oxytoca coinfection in mares experiencing placentitis and abortion. The present report describes an uncommon case of abortion caused by R. equi and K. oxytoca coinfection in a thoroughbred mare in Brazil, in which the virulence profile of R. equi was investigated. Case Presentation: A 6-year-old thoroughbred mare, in her first pregnancy, showed increased uteroplacental junction and suspected placentitis on transabdominal ultrasound. The mare aborted a 10-month-old female fetus. A field necropsy was performed and hydrallantois was diagnosed. Conclusion: Although we isolated an avirulent strain of R. equi, coinfection of R. equi and K. oxytoca in the placenta and uterus probably potentiated the pathogenic effect of these micro-organisms, resulting in abortion.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Universidade Federal de Santa Maria (UFSM), Departamento de Medicina Veterinária Preventiva, Santa Maria, RS, BrasilUniversidade Federal de Santa Maria (UFSM), Departamento de Patologia, Santa Maria, RS, BrasilUniversidade Estadual Paulista Júlio de Mesquita Filho (UNESP), Faculdade de Medicina Veterinária e Zootecnia (FMVZ), Departamento de Higiene Veterinária e Saúde Pública, Botucatu, SP, BrasilUniversidade Estadual Paulista Júlio de Mesquita Filho (UNESP), Faculdade de Medicina Veterinária e Zootecnia (FMVZ), Departamento de Higiene Veterinária e Saúde Pública, Botucatu, SP, Brasil