Respiratory disorders are responsible for poor performance in racehorses and for overall significant economic problems in the equine industry (1). The contribution of bacteria belonging to the genus Mycoplasma, that have been reported several times, in the observed clinical signs has yet to be clarified (2 – 3). This was the purpose of my thesis. We started by optimising tools for detection, isolation and identification of equine respiratory Mycoplasma species using spiked samples and serial dilution of DNA. A preliminary test showed that out of 616 respiratory samples, 104 were positive for mycoplasmas, of which 96% belong to the species Mycoplasma (M.) equirhinis, more regularly detected in tracheal washes than in bronchoalveolar lavages and in racehorses than others (4). The clinical implication of M. equirhinis was further studied on an enlarged set of 1948 clinical samples through establishing a clinical score and analysing risk factors for detection. In contrast to the pattern observed for bacterial pathogens or opportunists, the prevalence of M. equirhinis remained constant throughout the clinical score of the samples. Nonetheless, in presence of Streptococcus equi subs. zooepidemicus (S. zoo) or viruses like EHV-5 the prevalence of M. equirhinis was significantly increased. Furthermore, co-infection with S. zoo was associated with, on average, a significantly higher load of S. zoo. These results indicate that M. equirhinis is certainly not a pathogen but could contribute to clinical signs in association with other viruses or bacteria.