The correct diagnosis and treatment of disease in chambered nautiluses Nautilus pompilius are difficult. The basic morphology and natural behaviors of the species tend to mask the initial onset of a disease or infection, and by the time it is observable, mortality usually occurs. A single specimen of chambered nautilus at the Museum of Nature South Tyrol, Italy, exhibited symptoms of an infection, including mucus production, abnormal buoyancy, irregular hood appearance, and inconsistent tentacle movement. Soon after, tetracycline hydrochloride was administered in bath form at 25 mg/L in seawater daily for 30 min over six consecutive days. The specimen regained normal buoyancy after the fifth day. After the 6‐d treatment, the specimen regained all normal behaviors and appearance within 2 weeks. More than 1 year after treatment, the specimen was exhibiting normal behaviors with no adverse reactions.