Background: Seabather's eruption (SBE) is a highly pruritic dermatosis affecting swimmers and divers in marine waters off Florida, in the Gulf of Mexico, and the Caribbean Sea. Its cause has been attributed to various organisms but recently to the larvae of the schyphomedusa, Linuche unguiculata. Objective: We attempted to determine whether immature and adult Linuche cause SBE. Methods: Episodes of SBE in the Cancun and Cozumel area of the Mexican Caribbean were evaluated during the season of high tourism (January-June). This time corresponds to the moments in the life cycle when the three swimming stages of L unguiculata—ephyrae, medusae, and larvae—can be sequentially observed. Our methods include (1) observations by divers, biologists, and students coinciding with stinging outbreaks and the onset of SBE; (2) serologic evaluation of individuals stung by L unguiculata; and (3) the demonstration of Linuche nematocysts on the affected skin. Results: All 3 swimming Linuche stages can cause SBE. Conclusion: The offending stages of Linuche can be identified by the cutaneous lesion's morphology and the time of year. (J Am Acad Dermatol 2001;44:624-8.)