Before the 1970s, the United States sometimes disposed of at sea excess, obsolete, or unserviceable munitions, including chemical munitions. Chemical munitions known to have been disposed of at sea included munitions filled with sulfur mustard, a vesicant (i.e., an agent that causes chemical burns or blisters of the skin and mucous membranes. Signs and symptoms of exposure to a mustard agent can include redness and blistering of the skin, eye irritation, rhinorrhea, hoarseness, shortness of breath, and (rarely) diarrhea and abdominal discomfort. Since 2004, CDC has received notification of three separate incidents of exposure to sulfur mustard munitions. In one incident, a munition was found with ocean-dredged marine shells used to pave a driveway. The other two incidents involved commercial clam fishing operations. This report highlights the importance of considering exposure to sulfur mustard in the differential diagnosis of signs and symptoms compatible with exposure to a vesicant agent, especially among persons involved with clam fishing or sea dredging operations.