In June of 2016, an 11-year old Standardbred mare presented to Cornell’s Large Animal Medicine Service for evaluation of a fever of unknown origin and a twelve-hour history of lethargy and inappetence. Initial diagnostics included point of care bloodwork and ultrasound of both the thoracic and abdominal cavities. An increased amount and echogenicity of free abdominal fluid was noted and subsequently sampled. With the characteristic presentation and abnormalities found on abdominal fluid analysis, a preliminary diagnosis of Actinobacillus peritonitis was made and treatment was initiated. This diagnosis was later confirmed by positive culture of Actinobacillus spp. in the abdominal fluid. The focuses of this case include the classic presentation of Actinobacillus equuli peritonitis in horses, characteristic results of a diagnostic workup, the standard approach and rationale for treatment, and discussion of the rare incidence of and the possible pathogenesis of this disease.