A 10-yr-old female okapi (Okapia johnstoni) at the San Diego Zoo Safari Park was evaluated for intermittent malaise, inappetence, occasional cough, abdominal splinting, and licking at both flanks. Physical examination revealed tachypnea, tachycardia, and fluid sounds on thoracic auscultation. Transthoracic ultrasound showed multiple uniform, anechoic filled structures in the right and left pleural space. Surgical exploration of the thoracic cavity revealed bilateral, mature, fibrous, compartmentalizing adhesions between the visceral and parietal pleura, confirming a diagnosis of chronic, infectious, fibrinous pleuritis. The suspected etiology was occult aspiration pneumonia secondary to historical episodes of regurgitation associated with general anesthesia. Culture of the pleural fluid and fibrous adhesions grew Trueperella (Arcanobacterium) pyogenes, Arcanobacterium haemolyticum, and few Fusobacterium species. Treatment consisted of chest-tube placement to establish drainage, thoracic lavage, unilateral surgical debridement, and long-term antibiotics. The animal made a complete clinical recovery over 7 mo.