Background: Haloclava producta, the “ghost anemone”, is a burrowing sea anemone in estuarine sediments of the US East Coast and Gulf of Mexico. It has never been identified as harmful to human beings; however, residents of Long Island, New York develop a pruritic, erythematous, vesiculopapular dermatitis on areas of the body that contact these organisms. Neither the condition nor its cause has been described in the medical literature. Methods: We reviewed information of all water-related dermatitis reported by beach personnel, health providers, and affected swimmers to the Office of Marine Ecology, Nassau County Department of Health, New York from 1970 to 1991. Several episodes of an unfamiliar dermatitis among clam diggers, first recognized in 1981, initiated sampling efforts in 1985 when one victim, a bay constable, identified the area he frequented. During 1991, >100 persons were affected; sampling continued during reported cases. H. producta was isolated from all suspect sediment samples and tested on healthy subjects. Results: Typical of the phylum Cnidaria, H. producta has nematocysts capable of inflicting the observed dermatitis. Pressing them to the skin of healthy subjects produces dermatitis indistinguishable from that of victims. No other species with the same capability were identified from samples. Conclusion: H. producta is the apparent causative agent of ghost anemone dermatitis. Accurate diagnosis will allow appropriate treatment, education regarding prevention, and avoidance of unnecessary diagnostic tests or antibiotic use. (J Am Acad Dermatol 2002;47:722-6.)