A 3 month old 120 kg thoroughbred colt presented with a chief complaint of diarrhea, depression, fever, and inappetence with no history of respiratory illness. He was treated but his signs did not resolve and he succumbed to his disease. Postmortem findings included severe, diffuse chronic pyogranulomatous abdominal lymphadenitis, lymphangitis, and lymphangiectasia with severe, diffuse chronic neocroulcerative pyogranulomatous enterocolitis and typhlitis, and severe, chronic multifocal pyogranulomatous bronchopneumonia. The presumptive diagnosis was Rhodococcus equi, which was confirmed with bacteriology and histology. Rhodococcus equi (R. equi) is a gram-positive facultative anaerobic bacterium, characterized as an opportunistic pathogen that lives and replicates within host macrophages. This bacteria?s ability to retain viability within macrophages may be due to a delay in maturation of phagosomes to phagolysosomes. Although most horse farms contain R. equi in the enivronment, several factors contribute to the role of this bacterium as a pathogen. Since complete prevention is not possible, early detection and treatment is critical. The treatment of choice is a combination of a macrolide (erythromycin, clarithromycin or azithromycin) with rifampin.